feat(#955): split agent page serving — statics from nix store, API proxied

gateway_nginx.rs reads HIVE_AGENT_FRONTEND_DIR (injected by hive-c0re.nix
as ${cfg.frontend}/agent). When set, agents.conf emits per-agent split
blocks instead of the old single proxy_pass:

  # Compiled assets — immutable nix store path, cache 1y
  location ^~ /agent/<name>/static/ {
      alias <frontend>/static/;
      expires 1y; add_header Cache-Control "public, immutable, ...";
  }
  # Static dist + proxy fallback
  location /agent/<name>/ {
      alias <frontend>/;
      try_files $uri $uri.html $uri/index.html @<name>_dynamic;
  }
  location @<name>_dynamic {
      proxy_pass <upstream>;   # api, events, icon, login, …
      …proxy headers unchanged…
  }

try_files path resolution (nginx applies alias mapping first):
  $uri           — exact file (/static/app.js → static/app.js)
  $uri.html      — bare-path fallback (/stats → stats.html)
  $uri/index.html — directory index (/ → index.html)
  @<name>_dynamic — proxy catchall for anything not in the dist

Adding pages to the frontend dist works automatically — no generator
change needed. Per-agent extraFiles (in mergedDist, not in the base
nix-store path) continue to proxy to the agent daemon.

frontend is a nix store path injected at build time — only [a-z0-9/._-],
no shell metacharacters — safe to interpolate without sanitization;
comment added documenting this assumption.

Without HIVE_AGENT_FRONTEND_DIR the existing single-proxy block is
emitted unchanged — backward-compatible for deployments without the env.

render() takes frontend_dir as a parameter so tests exercise both code
paths safely in parallel. 13 tests: 7 legacy, 6 split-mode. No clippy
warnings in changed files.

nix/modules/hive-c0re.nix: inject HIVE_AGENT_FRONTEND_DIR = "${cfg.frontend}/agent".

hive-c0re/src/gateway_nginx.rs

@@ -40,9 +40,18 @@ const PROXY_HEADER_BLOCK: &str = "    proxy_http_version 1.1;
     proxy_set_header X-Forwarded-Proto $scheme;";
 
 /// Render the nginx include body for `names`. Manager is filtered out.
-/// Each remaining agent gets a `location /agent/<name>/` block whose
+///
-/// `proxy_pass` targets the unix socket when the `.bound` marker
+/// When `frontend_dir` is `Some(path)` each agent gets split location
-/// exists, or TCP loopback otherwise.
+/// blocks:
+/// - Exact HTML pages (`/`, `/stats`, `/screen`) → `alias` from the nix
+///   store dist. These load even while the agent daemon is restarting.
+/// - Static assets (`/static/`) → `alias` from the nix store with cache
+///   headers. Served by nginx directly, no agent socket round-trip.
+/// - Everything else (all `/api/*`, `/events/*`, `/screen/ws`, `/icon`,
+///   `/send`, `/login/*`) → proxied to the agent daemon as before.
+///
+/// When `frontend_dir` is `None` (legacy) each agent gets a single
+/// `location /agent/<name>/` proxy block.
 ///
 /// Output is deterministic for the same (sorted) set of names +
 /// `.bound` state: no timestamps, no UUIDs. `BTreeMap` would give
@@ -50,7 +59,7 @@ const PROXY_HEADER_BLOCK: &str = "    proxy_http_version 1.1;
 /// they care about diff stability, but the gateway treats the blocks
 /// as unordered by nginx's longest-prefix-match rules so ordering only
 /// affects human readability.
-fn render(names: &[String]) -> String {
+fn render(names: &[String], frontend_dir: Option<&str>) -> String {
     let mut out = String::from(
         "# Generated by hive-c0re \u{2014} do not edit.\
          \n# Refreshed on every topology change + when agents bind/drop their unix sockets.\
@@ -69,18 +78,69 @@ fn render(names: &[String]) -> String {
         } else {
             format!("http://127.0.0.1:{port}/")
         };
-        let _ = write!(
+
-            out,
+        if let Some(frontend) = frontend_dir {
-            "\nlocation /agent/{name}/ {{\n\
+            // Split mode: try to serve files from the nix-store dist first;
-             {PROXY_HEADER_BLOCK}\n\
+            // fall through to the agent daemon for anything not found there.
-             \n    proxy_pass {upstream};\n\
+            //
-             \n    proxy_set_header X-Forwarded-Prefix /agent/{name};\n\
+            // `frontend` is a nix store path injected at build time by hive-c0re.nix
-             \n    proxy_buffering off;\n\
+            // (`HIVE_AGENT_FRONTEND_DIR = "${cfg.frontend}/agent"`). Nix store paths
-             \n    proxy_read_timeout 1d;\n\
+            // are of the form `/nix/store/<hash>-<name>` — only [a-z0-9/._-], never
-             \n    proxy_intercept_errors on;\n\
+            // shell metacharacters — so interpolating directly into the nginx config
-             \n    error_page 502 503 504 = /__hive_agent_unreachable;\n\
+            // string is safe. We do NOT accept user-controlled input here.
-             }}\n",
+            //
-        );
+            // Location priority for a request to /agent/<name>/...:
+            //   ^~ /agent/<name>/static/ — highest priority; matches compiled assets.
+            //      Nix store paths are content-addressed and immutable — cache forever.
+            //   /agent/<name>/  — try_files from dist, proxy fallback for the rest.
+            //
+            // try_files resolution (nginx applies the alias mapping before checking):
+            //   $uri            — exact file match (/static/app.js → static/app.js)
+            //   $uri.html       — bare-path .html fallback (/stats → stats.html)
+            //   $uri/index.html — directory index (/ → index.html)
+            //   @<name>_dynamic — proxy catchall: api, events, icon, login, …
+            //
+            // Adding pages to the frontend dist works automatically — no generator
+            // change needed. Per-agent extraFiles (in mergedDist, not in the base
+            // dist path) still proxy to the agent daemon via @<name>_dynamic.
+            let _ = write!(
+                out,
+                "\n# Compiled JS/CSS assets — content-addressed nix store path, cache forever\n\
+                 location ^~ /agent/{name}/static/ {{\n\
+                 \n    alias {frontend}/static/;\n\
+                 \n    expires 1y;\n\
+                 \n    add_header Cache-Control \"public, immutable, max-age=31536000\";\n\
+                 }}\n\
+                 \n# Static dist + proxy fallback for dynamic paths\n\
+                 location /agent/{name}/ {{\n\
+                 \n    alias {frontend}/;\n\
+                 \n    try_files $uri $uri.html $uri/index.html @{name}_dynamic;\n\
+                 }}\n\
+                 \nlocation @{name}_dynamic {{\n\
+                 {PROXY_HEADER_BLOCK}\n\
+                 \n    proxy_pass {upstream};\n\
+                 \n    proxy_set_header X-Forwarded-Prefix /agent/{name};\n\
+                 \n    proxy_buffering off;\n\
+                 \n    proxy_read_timeout 1d;\n\
+                 \n    proxy_intercept_errors on;\n\
+                 \n    error_page 502 503 504 = /__hive_agent_unreachable;\n\
+                 }}\n",
+            );
+        } else {
+            // Legacy mode: proxy everything to the agent daemon.
+            let _ = write!(
+                out,
+                "\nlocation /agent/{name}/ {{\n\
+                 {PROXY_HEADER_BLOCK}\n\
+                 \n    proxy_pass {upstream};\n\
+                 \n    proxy_set_header X-Forwarded-Prefix /agent/{name};\n\
+                 \n    proxy_buffering off;\n\
+                 \n    proxy_read_timeout 1d;\n\
+                 \n    proxy_intercept_errors on;\n\
+                 \n    error_page 502 503 504 = /__hive_agent_unreachable;\n\
+                 }}\n",
+            );
+        }
     }
     out
 }
@@ -104,7 +164,10 @@ fn render(names: &[String]) -> String {
 /// housekeeping restart or the next manual reload; the host's agent
 /// topology has already been written correctly.
 pub fn write(names: &[String]) -> Result<()> {
-    let body = render(names);
+    let frontend_dir = std::env::var("HIVE_AGENT_FRONTEND_DIR")
+        .ok()
+        .filter(|s| !s.is_empty());
+    let body = render(names, frontend_dir.as_deref());
     let path = host_conf_path();
     if std::fs::read_to_string(&path).ok().as_deref() == Some(&body) {
         return Ok(());
@@ -173,9 +236,11 @@ fn reload_gateway_nginx() {
 mod tests {
     use super::*;
 
+    // ── legacy mode (frontend_dir = None) ──────────────────────────────
+
     #[test]
     fn render_empty_is_header_only() {
-        let body = render(&[]);
+        let body = render(&[], None);
         assert!(body.starts_with("# Generated by hive-c0re"));
         // No location blocks when no agents.
         assert!(!body.contains("location"));
@@ -187,9 +252,7 @@ mod tests {
             .iter()
             .map(|s| (*s).to_owned())
             .collect();
-        // Use build_map_with so we don't need real .bound files.
+        let body = render(&names, None);
-        // For render, we call it directly with names.
-        let body = render(&names);
         assert!(!body.contains(&format!("/agent/{MANAGER_NAME}/")));
         assert!(body.contains("/agent/iris/"));
     }
@@ -198,7 +261,7 @@ mod tests {
     fn render_tcp_upstream_when_no_bound_marker() {
         // No .bound file on disk → falls back to TCP loopback.
         let names = vec!["iris".to_owned()];
-        let body = render(&names);
+        let body = render(&names, None);
         let port = lifecycle::agent_web_port("iris");
         assert!(
             body.contains(&format!("proxy_pass http://127.0.0.1:{port}/")),
@@ -209,7 +272,7 @@ mod tests {
     #[test]
     fn render_includes_proxy_headers() {
         let names = vec!["iris".to_owned()];
-        let body = render(&names);
+        let body = render(&names, None);
         // All required proxy headers present (WebSocket upgrade + forwarded-for).
         assert!(body.contains("proxy_http_version 1.1"));
         assert!(body.contains("proxy_set_header Upgrade $http_upgrade"));
@@ -219,7 +282,7 @@ mod tests {
     #[test]
     fn render_includes_forwarded_prefix() {
         let names = vec!["atlas".to_owned()];
-        let body = render(&names);
+        let body = render(&names, None);
         assert!(
             body.contains("X-Forwarded-Prefix /agent/atlas"),
             "expected X-Forwarded-Prefix for atlas, got:\n{body}"
@@ -229,7 +292,7 @@ mod tests {
     #[test]
     fn render_includes_error_pages() {
         let names = vec!["iris".to_owned()];
-        let body = render(&names);
+        let body = render(&names, None);
         assert!(body.contains("proxy_intercept_errors on"));
         assert!(body.contains("__hive_agent_unreachable"));
     }
@@ -237,8 +300,95 @@ mod tests {
     #[test]
     fn render_includes_buffering_and_timeout() {
         let names = vec!["iris".to_owned()];
-        let body = render(&names);
+        let body = render(&names, None);
         assert!(body.contains("proxy_buffering off"));
         assert!(body.contains("proxy_read_timeout 1d"));
     }
+
+    // ── split mode (frontend_dir = Some) ───────────────────────────────
+
+    const FAKE_FRONTEND: &str = "/nix/store/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-frontend/agent";
+
+    #[test]
+    fn split_mode_has_immutable_cache_for_static_assets() {
+        let names = vec!["iris".to_owned()];
+        let body = render(&names, Some(FAKE_FRONTEND));
+        // ^~ prefix block for /static/ with immutable cache headers
+        assert!(
+            body.contains("location ^~ /agent/iris/static/ {"),
+            "expected ^~ static location, got:\n{body}"
+        );
+        assert!(body.contains(&format!("alias {FAKE_FRONTEND}/static/;")));
+        assert!(body.contains("expires 1y;"));
+        assert!(body.contains("immutable"));
+    }
+
+    #[test]
+    fn split_mode_has_alias_and_try_files() {
+        let names = vec!["iris".to_owned()];
+        let body = render(&names, Some(FAKE_FRONTEND));
+        // Plain prefix location with alias + try_files
+        assert!(
+            body.contains("location /agent/iris/ {"),
+            "expected try_files location, got:\n{body}"
+        );
+        assert!(
+            body.contains(&format!("alias {FAKE_FRONTEND}/;")),
+            "expected alias, got:\n{body}"
+        );
+        // try_files handles exact file, bare-path .html, directory index
+        assert!(
+            body.contains("try_files $uri $uri.html $uri/index.html @iris_dynamic"),
+            "expected try_files with .html fallback, got:\n{body}"
+        );
+    }
+
+    #[test]
+    fn split_mode_has_named_proxy_location() {
+        let names = vec!["iris".to_owned()];
+        let body = render(&names, Some(FAKE_FRONTEND));
+        // Named location for dynamic proxy
+        assert!(
+            body.contains("location @iris_dynamic {"),
+            "expected named dynamic location, got:\n{body}"
+        );
+        assert!(body.contains("proxy_pass"));
+        assert!(body.contains("proxy_intercept_errors on"));
+        assert!(body.contains("__hive_agent_unreachable"));
+    }
+
+    #[test]
+    fn split_mode_filters_manager() {
+        let names: Vec<String> = [MANAGER_NAME, "iris"]
+            .iter()
+            .map(|s| (*s).to_owned())
+            .collect();
+        let body = render(&names, Some(FAKE_FRONTEND));
+        assert!(!body.contains(&format!("/agent/{MANAGER_NAME}/")));
+        assert!(body.contains("/agent/iris/"));
+    }
+
+    #[test]
+    fn split_mode_proxy_includes_forwarded_prefix() {
+        let names = vec!["atlas".to_owned()];
+        let body = render(&names, Some(FAKE_FRONTEND));
+        assert!(
+            body.contains("X-Forwarded-Prefix /agent/atlas"),
+            "expected X-Forwarded-Prefix for atlas in split mode, got:\n{body}"
+        );
+    }
+
+    #[test]
+    fn split_mode_named_location_uses_agent_name() {
+        // Named location must be unique per agent to avoid nginx conflicts
+        let names = vec!["damocles".to_owned(), "iris".to_owned()];
+        let body = render(&names, Some(FAKE_FRONTEND));
+        assert!(body.contains("@damocles_dynamic"));
+        assert!(body.contains("@iris_dynamic"));
+        // Each agent gets exactly one named dynamic location block.
+        let damocles_count = body.matches("@damocles_dynamic {").count();
+        let iris_count = body.matches("@iris_dynamic {").count();
+        assert_eq!(damocles_count, 1, "expected exactly one damocles_dynamic block");
+        assert_eq!(iris_count, 1, "expected exactly one iris_dynamic block");
+    }
 }

nix/modules/hive-c0re.nix

@@ -302,6 +302,14 @@ in
         # serves this via `tower_http::ServeDir` for any path it doesn't
         # match against an API/action route.
         HIVE_STATIC_DIR = "${cfg.frontend}/dashboard";
+        # Path to the base agent frontend dist. hive-c0re's
+        # gateway_nginx.rs uses this to generate split location
+        # blocks in agents.conf — static HTML/CSS/JS served from the
+        # nix store directly; dynamic API paths still proxied to the
+        # agent daemon. The nix store is shared across nspawn
+        # containers, so this path is reachable from inside the
+        # gateway container's nginx.
+        HIVE_AGENT_FRONTEND_DIR = "${cfg.frontend}/agent";
         # Path to the static runtime asset tree (branding + claude
         # prompts). `hive_sh4re::assets::*` reads paths underneath.
         # `forge.rs` reads the avatar PNGs from here on startup.