refactor(#2693): null, not "", for the unpinned claude-code

mara on PR #2769: "make the default null instead of special casing """.

`claude_code_path` was a `String` whose empty value meant "no host-level
pin". That is a sentinel doing an `Option`'s job — the same shape argus
and mara already rejected on #2755's weights, and the same
empty-field cruft mara called out on #2756.

So it is `Option<String>` end to end:

- host module: `claudeCodePath` evaluates to `null` when
  `claudeCodePackage` is unset, so `serve.json` carries JSON `null`
  rather than `""`.
- `Coordinator` + `HiveEnv`: `Option<String>`, defaulting to `None`.
- `render_flake`/`render_flake_with_lookup`: `Option<&str>`, and the
  emission is an `if let Some(path)` instead of an `is_empty()` guard.
- agent module: `hyperhive.claudeCodePath` is `nullOr str`, default
  `null`.

Behaviour is unchanged in both directions; only the way "unset" is
spelled moves. The `builtins.hasContext` assertion still guards the
pinned case (short-circuited by the null check, so an unpinned hive
never evaluates it).

16/16 `meta::` tests, clippy clean, `nix fmt` no-op, `nix build .#docs`
green.

hive-c0re/src/coordinator.rs

@@ -65,14 +65,14 @@ pub struct Coordinator {
     /// Store path of the `claude-code` build every agent runs, written
     /// into each per-agent flake as `hyperhive.claudeCodePath`. Set by
     /// the NixOS module option `services.hyperhive.c0re.claudeCodePackage`
-    /// (which resolves the package and hands us its path). Empty string =
-    /// every agent keeps the `claude-code` from its own nixpkgs.
+    /// (which resolves the package and hands us its path). `None` = every
+    /// agent keeps the `claude-code` from its own nixpkgs.
     ///
     /// A path rather than a flake input because containers share the
     /// host's `/nix/store`: the binary is already reachable inside them,
     /// closure and all. The host module is what keeps it from being
     /// garbage-collected — see that option.
-    pub claude_code_path: String,
+    pub claude_code_path: Option<String>,
     /// TCP port the host's hive-c0re dashboard listens on. Inlined into
     /// each per-agent flake so the agent's web UI can build the right
     /// rebuild-button URL pointing back at the dashboard.
@@ -224,11 +224,11 @@ pub struct HiveEnv {
     /// so a doc edit only re-locks this input, not the whole source.
     pub hyperhive_docs_flake: String,
     pub nixpkgs_flake: String,
-    /// Store path of the `claude-code` agents run, or empty for "each
+    /// Store path of the `claude-code` agents run, or `None` for "each
     /// agent keeps the one out of its own nixpkgs". Travels into the
     /// container as `hyperhive.claudeCodePath` — a plain string, kept
     /// alive host-side by the module that resolved it.
-    pub claude_code_path: String,
+    pub claude_code_path: Option<String>,
     pub dashboard_port: u16,
     pub operator_pronouns: String,
     pub context_window_tokens: std::collections::HashMap<String, u64>,
@@ -251,7 +251,7 @@ impl Default for HiveEnv {
             hyperhive_flake: "/etc/hyperhive".to_string(),
             hyperhive_docs_flake: String::new(),
             nixpkgs_flake: String::new(),
-            claude_code_path: String::new(),
+            claude_code_path: None,
             dashboard_port: 7000,
             operator_pronouns: "she/her".to_string(),
             context_window_tokens: std::collections::HashMap::from([

hive-c0re/src/meta.rs

@@ -92,7 +92,7 @@ pub async fn sync_agents(hive: &HiveEnv, agents: &[AgentSpec]) -> Result<()> {
         &hive.hyperhive_flake,
         &hive.hyperhive_docs_flake,
         &hive.nixpkgs_flake,
-        &hive.claude_code_path,
+        hive.claude_code_path.as_deref(),
         hive.dashboard_port,
         &hive.operator_pronouns,
         &hive.context_window_tokens,
@@ -651,7 +651,7 @@ fn render_flake(
     hyperhive_flake: &str,
     docs_flake: &str,
     nixpkgs_flake: &str,
-    claude_code_path: &str,
+    claude_code_path: Option<&str>,
     dashboard_port: u16,
     operator_pronouns: &str,
     context_window_tokens: &std::collections::HashMap<String, u64>,
@@ -940,7 +940,7 @@ fn render_flake_with_lookup<F>(
     hyperhive_flake: &str,
     docs_flake: &str,
     nixpkgs_flake: &str,
-    claude_code_path: &str,
+    claude_code_path: Option<&str>,
     dashboard_port: u16,
     operator_pronouns: &str,
     context_window_tokens: &std::collections::HashMap<String, u64>,
@@ -1063,12 +1063,9 @@ where
     // which pure eval rejects. The agent module puts its `bin/` on the
     // harness PATH; the host module holds the gc root, since a path
     // spelled out here is text and references nothing.
-    // Empty = no override; agents keep their own nixpkgs' `claude-code`.
-    if !claude_code_path.is_empty() {
-        let _ = writeln!(
-            out,
-            "              hyperhive.claudeCodePath = \"{claude_code_path}\";"
-        );
+    // `None` = no override; agents keep their own nixpkgs' `claude-code`.
+    if let Some(path) = claude_code_path {
+        let _ = writeln!(out, "              hyperhive.claudeCodePath = \"{path}\";");
     }
     // CA trust: embed every hive-trusted CA so each agent validates them at
     // build time. The list is the hive's own self-signed CA (when active)
@@ -1580,7 +1577,7 @@ mod tests {
             "github:example/hyperhive",
             "path:/nix/store/bbbb-hyperhive-docs-source",
             "path:/nix/store/aaaa-nixpkgs-source",
-            "",
+            None,
             8000,
             "she/her",
             &std::collections::HashMap::new(),
@@ -1625,7 +1622,7 @@ mod tests {
             "github:example/hyperhive",
             "",
             "path:/nix/store/aaaa-nixpkgs-source",
-            "",
+            None,
             8000,
             "she/her",
             &std::collections::HashMap::new(),
@@ -1649,7 +1646,7 @@ mod tests {
             "github:example/hyperhive",
             "path:/nix/store/bbbb-hyperhive-docs-source",
             "path:/nix/store/aaaa-nixpkgs-source",
-            "/nix/store/cccc-claude-code-2.1.220",
+            Some("/nix/store/cccc-claude-code-2.1.220"),
             8000,
             "she/her",
             &std::collections::HashMap::new(),
@@ -1668,7 +1665,7 @@ mod tests {
 
     #[test]
     fn render_flake_omits_claude_path_when_unset() {
-        // Empty = no host-level pin: the option is left undefined so the
+        // `None` = no host-level pin: the option is left undefined so the
         // agent module keeps its own nixpkgs' `claude-code` (and keeps it
         // in `environment.systemPackages`, which is what makes the
         // unpinned case self-contained).
@@ -1676,7 +1673,7 @@ mod tests {
             "github:example/hyperhive",
             "path:/nix/store/bbbb-hyperhive-docs-source",
             "path:/nix/store/aaaa-nixpkgs-source",
-            "",
+            None,
             8000,
             "she/her",
             &std::collections::HashMap::new(),
@@ -1697,7 +1694,7 @@ mod tests {
             "github:example/hyperhive",
             "path:/nix/store/bbbb-hyperhive-docs-source",
             "",
-            "",
+            None,
             8000,
             "she/her",
             &std::collections::HashMap::new(),
@@ -1728,7 +1725,7 @@ mod tests {
             "github:example/hyperhive",
             "path:/nix/store/bbbb-hyperhive-docs-source",
             "path:/nix/store/aaaa-nixpkgs-source",
-            "",
+            None,
             8000,
             "she/her",
             &std::collections::HashMap::new(),
@@ -1762,7 +1759,7 @@ mod tests {
             "github:example/hyperhive",
             "path:/nix/store/bbbb-hyperhive-docs-source",
             "path:/nix/store/aaaa-nixpkgs-source",
-            "",
+            None,
             8000,
             "she/her",
             &std::collections::HashMap::new(),
@@ -1798,7 +1795,7 @@ mod tests {
             "github:example/hyperhive",
             "path:/nix/store/bbbb-hyperhive-docs-source",
             "path:/nix/store/aaaa-nixpkgs-source",
-            "",
+            None,
             8000,
             "she/her",
             &std::collections::HashMap::new(),
@@ -1837,7 +1834,7 @@ mod tests {
             "github:example/hyperhive",
             "path:/nix/store/bbbb-hyperhive-docs-source",
             "path:/nix/store/aaaa-nixpkgs-source",
-            "",
+            None,
             8000,
             "she/her",
             &std::collections::HashMap::new(),
@@ -1880,7 +1877,7 @@ mod tests {
                 "github:example/hyperhive",
                 "path:/nix/store/bbbb-hyperhive-docs-source",
                 "path:/nix/store/aaaa-nixpkgs-source",
-                "",
+                None,
                 8000,
                 "she/her",
                 &std::collections::HashMap::new(),
@@ -1964,7 +1961,7 @@ mod tests {
                 "github:example/hyperhive",
                 "path:/nix/store/bbbb-hyperhive-docs-source",
                 "path:/nix/store/aaaa-nixpkgs-source",
-                "",
+                None,
                 8000,
                 "she/her",
                 &std::collections::HashMap::new(),
@@ -2050,7 +2047,7 @@ mod tests {
                 "github:example/hyperhive",
                 "path:/nix/store/bbbb-hyperhive-docs-source",
                 "path:/nix/store/aaaa-nixpkgs-source",
-                "",
+                None,
                 8000,
                 "she/her",
                 &std::collections::HashMap::new(),
@@ -2093,7 +2090,7 @@ mod tests {
             "github:example/hyperhive",
             "path:/nix/store/bbbb-hyperhive-docs-source",
             "path:/nix/store/aaaa-nixpkgs-source",
-            "",
+            None,
             8000,
             "she/her",
             &std::collections::HashMap::new(),

nix/agent-modules/default.nix

@@ -62,11 +62,11 @@
   };
 
   options.hyperhive.claudeCodePath = lib.mkOption {
-    type = lib.types.str;
-    default = "";
+    type = lib.types.nullOr lib.types.str;
+    default = null;
     example = "/nix/store/…-claude-code-2.1.220";
     description = ''
-      Store path of the `claude-code` this agent runs, or `""` (the
+      Store path of the `claude-code` this agent runs, or `null` (the
       default) to use the `claude-code` from the container's own
       nixpkgs.
 
@@ -212,7 +212,7 @@
     ]
     ++ [
       (
-        if config.hyperhive.claudeCodePath == "" then
+        if config.hyperhive.claudeCodePath == null then
           pkgs.claude-code
         else
           # Host-pinned claude: a symlink farm around a path the

nix/host-modules/hive-c0re/default.nix

@@ -59,7 +59,7 @@ let
     fi
   '';
 
-  # Store path of the `claude-code` every agent runs, or "" for "each
+  # Store path of the `claude-code` every agent runs, or null for "each
   # agent keeps the one out of its own nixpkgs". meta.rs writes it into
   # each agent's generated flake as a plain string literal, and the agent
   # module puts its `bin/` on the harness PATH.
@@ -73,7 +73,7 @@ let
   # do not hand meta.rs the path by a route that drops it. The failure
   # mode is a garbage-collected `claude` and a hive that can't take a
   # turn, weeks after the commit that caused it.
-  claudeCodePath = if cfg.claudeCodePackage == null then "" else "${cfg.claudeCodePackage}";
+  claudeCodePath = if cfg.claudeCodePackage == null then null else "${cfg.claudeCodePackage}";
 
   # The `hive-c0re serve` config JSON. Keys are snake_case to match the
   # `ServeConfig` serde shape the daemon deserialises (the