rust+nix: load static assets at runtime, drop build.rs (#555)

Cuts every `include_bytes!`/`include_str!` of a non-rust path in
the workspace over to runtime file loads from `$HIVE_ASSETS_DIR`
(the `hyperhive-assets` derivation introduced in the previous
commit). After this commit the rust derivation has no compile-time
dependency on `branding/*` or `hive-ag3nt/prompts/*` anymore.

Call-site flips:

- `hive-c0re/src/forge.rs::CORE_AVATAR_PNG` /
  `CONFIG_ORG_AVATAR_PNG`: were `include_bytes!` of
  `branding/hyperhive.png` and `$OUT_DIR/agent-configs.png`. Now
  `ensure_core_avatar` / `ensure_config_org_avatar` `tokio::fs::read`
  via `hive_sh4re::assets::{core_avatar_png, config_org_avatar_png}`
  at startup. The `agent-configs.png` is now rendered by the
  `hyperhive-assets` derivation's rsvg-convert step (was
  `hive-c0re/build.rs` + librsvg on the rust derivation's
  nativeBuildInputs — both gone in the next commit).
- `hive-ag3nt/src/prompt.rs::TEMPLATE`: `render` now takes the
  template as an argument; `write_system_prompt` reads it once from
  `$HIVE_ASSETS_DIR/prompts/system.md` before calling render. The
  test module still `include_str!`s the production template so
  `cargo test --workspace` doesn't need `HIVE_ASSETS_DIR` set —
  this is the only remaining compile-time reference to the file
  from the rust workspace, gated to `#[cfg(test)]`.
- `hive-ag3nt/src/turn.rs::CLAUDE_SETTINGS`: was `include_str!`'d
  and written via `tokio::fs::write`; now `tokio::fs::copy` from
  `$HIVE_ASSETS_DIR/prompts/claude-settings.json` into the
  per-agent socket dir.
- `hive-ag3nt/src/web_ui.rs::DEFAULT_ICON`: was `include_str!`'d;
  now read on-demand from `$HIVE_ASSETS_DIR/branding/hyperhive.svg`
  inside `serve_icon`. Falls back to an empty body if missing so
  the endpoint never panics on a misconfigured container (matches
  the existing "per-agent icon.svg override" fallthrough).

`HIVE_ASSETS_DIR` wiring:

- Inside containers: `nix/templates/harness-base.nix`
  `environment.variables` sets it to
  `${pkgs.hyperhive-assets}/share/hyperhive` (resolved through
  the default overlay applied in `mkContainer`). Verified by
  building `agent-base-toplevel` and grepping the resulting
  `/etc/set-environment`.
- Host-side: `nix/modules/hive-c0re.nix` adds an `assets` option
  defaulting to `hyperhive.packages.${system}.assets`, threaded
  in from the flake's nixosModules wiring, and sets the same env
  var on the `hive-c0re` systemd unit so the daemon's
  `forge::ensure_*_avatar` startup hooks find the PNGs.

`hive-c0re/build.rs` deleted entirely; `[package].build` removed
from `hive-c0re/Cargo.toml`; rsvg-convert dependency lives in the
assets derivation only.

Validated: `nix build .#default .#checks.x86_64-linux.clippy
.#agent-base-toplevel .#manager-toplevel --fallback` all succeed.
`/etc/set-environment` in the toplevel shows
`HIVE_ASSETS_DIR="/nix/store/.../hyperhive-assets-0.1.0/share/hyperhive"`.

hive-ag3nt/src/prompt.rs

@@ -29,22 +29,24 @@
 
 use std::path::{Path, PathBuf};
 
-use anyhow::Result;
+use anyhow::{Context, Result};
 
 use crate::mcp::Flavor;
 
-const TEMPLATE: &str = include_str!("../prompts/system.md");
-
 /// Assemble the system prompt for a given flavor + label + pronouns.
 /// Pure function — no I/O. Splits out from [`write_system_prompt`] so
 /// the marker logic + substitution is unit-testable in isolation.
+/// The caller supplies the template body so tests can pass an inline
+/// fixture and production reads it once at harness startup via
+/// [`hive_sh4re::assets::prompt_template`] (`$HIVE_ASSETS_DIR/prompts/
+/// system.md`).
 #[must_use]
-pub fn render(flavor: Flavor, label: &str, operator_pronouns: &str) -> String {
+pub fn render(template: &str, flavor: Flavor, label: &str, operator_pronouns: &str) -> String {
     let target = match flavor {
         Flavor::Agent => "agent",
         Flavor::Manager => "manager",
     };
-    let body = filter_role_blocks(TEMPLATE, target);
+    let body = filter_role_blocks(template, target);
     body.replace("{label}", label)
         .replace("{operator_pronouns}", operator_pronouns)
 }
@@ -123,7 +125,11 @@ pub async fn write_system_prompt(
     let parent = socket.parent().unwrap_or_else(|| Path::new("/run/hive"));
     tokio::fs::create_dir_all(parent).await.ok();
     let pronouns = std::env::var("HIVE_OPERATOR_PRONOUNS").unwrap_or_else(|_| "she/her".to_owned());
-    let body = render(flavor, label, &pronouns);
+    let template_path = hive_sh4re::assets::prompt_template();
+    let template = tokio::fs::read_to_string(&template_path)
+        .await
+        .with_context(|| format!("read claude system prompt template from {}", template_path.display()))?;
+    let body = render(&template, flavor, label, &pronouns);
     let path = parent.join("claude-system-prompt.md");
     tokio::fs::write(&path, body).await?;
     tracing::info!(path = %path.display(), "wrote claude system prompt");
@@ -134,6 +140,18 @@ pub async fn write_system_prompt(
 mod tests {
     use super::*;
 
+    // #555: production reads the system-prompt template from
+    // `$HIVE_ASSETS_DIR/prompts/system.md` at startup. The test module
+    // still `include_str!`s it directly because:
+    //   1. the "real template still substitutes / still filters" tests
+    //      below need the actual production wording to be honest;
+    //   2. embedding it at compile time keeps `cargo test --workspace`
+    //      runnable without setting `HIVE_ASSETS_DIR`;
+    //   3. this is the ONLY remaining compile-time reference to
+    //      `prompts/system.md` from the rust workspace — production
+    //      code loads it at runtime.
+    const PRODUCTION_TEMPLATE: &str = include_str!("../prompts/system.md");
+
     const SAMPLE: &str = "\
 shared opener
 <!-- role:agent -->
@@ -233,7 +251,7 @@ shared closer
         // Real template's first agent line — keeps the renderer
         // honest about the {label} / {operator_pronouns} pair the
         // harness already relied on.
-        let rendered = render(Flavor::Agent, "alice", "they/them");
+        let rendered = render(PRODUCTION_TEMPLATE, Flavor::Agent, "alice", "they/them");
         assert!(rendered.contains("hyperhive agent `alice`"));
         assert!(rendered.contains("**they/them** pronouns"));
         assert!(!rendered.contains("{label}"));
@@ -246,7 +264,7 @@ shared closer
         // kill, schedule_*) MUST NOT appear in the agent's rendered
         // prompt. Drift between flavor and tool surface bites every
         // time it happens (cf. #511 missing-allow-list bug).
-        let rendered = render(Flavor::Agent, "alice", "she/her");
+        let rendered = render(PRODUCTION_TEMPLATE, Flavor::Agent, "alice", "she/her");
         assert!(!rendered.contains("request_init_config"));
         assert!(!rendered.contains("request_apply_commit"));
         assert!(!rendered.contains("get_logs"));
@@ -257,7 +275,7 @@ shared closer
 
     #[test]
     fn render_manager_includes_manager_only_tools() {
-        let rendered = render(Flavor::Manager, "hm1nd", "she/her");
+        let rendered = render(PRODUCTION_TEMPLATE, Flavor::Manager, "hm1nd", "she/her");
         assert!(rendered.contains("request_init_config"));
         assert!(rendered.contains("request_apply_commit"));
         assert!(rendered.contains("get_logs"));
@@ -269,9 +287,9 @@ shared closer
 
     #[test]
     fn render_uses_correct_role_opener() {
-        let agent = render(Flavor::Agent, "alice", "she/her");
+        let agent = render(PRODUCTION_TEMPLATE, Flavor::Agent, "alice", "she/her");
         assert!(agent.starts_with("You are hyperhive agent"));
-        let manager = render(Flavor::Manager, "hm1nd", "she/her");
+        let manager = render(PRODUCTION_TEMPLATE, Flavor::Manager, "hm1nd", "she/her");
         assert!(manager.starts_with("You are the hyperhive manager"));
     }
 }