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/turn.rs

@@ -10,7 +10,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
-use anyhow::{Result, bail};
+use anyhow::{Context, Result, bail};
 use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
 use tokio::process::Command;
 
@@ -18,15 +18,14 @@ use crate::events::{Bus, LiveEvent};
 use crate::login::LoginState;
 use crate::mcp;
 
-/// `--settings` JSON applied to every claude invocation. Lives as a
-/// properly-formatted file in `prompts/claude-settings.json` so it's easy
-/// to read and edit; we ship it via `include_str!`. We turn off claude's
-/// in-session auto-compaction and its cross-session auto-memory because
-/// hyperhive owns those concerns (`/compact` on overflow, notes
-/// persistence under `/state`). Unknown keys are silently ignored by
-/// claude-code; if a key gets renamed we'll spot it because the
-/// corresponding behavior will start firing mid-turn again.
-const CLAUDE_SETTINGS: &str = include_str!("../prompts/claude-settings.json");
+// `--settings` JSON is read at runtime from
+// `$HIVE_ASSETS_DIR/prompts/claude-settings.json` via
+// `hive_sh4re::assets::claude_settings()` (#555). We turn off claude's
+// in-session auto-compaction and its cross-session auto-memory because
+// hyperhive owns those concerns (`/compact` on overflow, notes
+// persistence under `/state`). Unknown keys are silently ignored by
+// claude-code; if a key gets renamed we'll spot it because the
+// corresponding behavior will start firing mid-turn again.
 
 /// Regex-ish marker claude-code emits when context overflows. Same string
 /// bitburner-agent watches for. Empirically reliable across claude-code
@@ -159,7 +158,14 @@ pub async fn write_settings(socket: &Path) -> Result<PathBuf> {
     let parent = socket.parent().unwrap_or_else(|| Path::new("/run/hive"));
     tokio::fs::create_dir_all(parent).await.ok();
     let path = parent.join("claude-settings.json");
-    tokio::fs::write(&path, CLAUDE_SETTINGS).await?;
+    // #555: source-of-truth is `$HIVE_ASSETS_DIR/prompts/claude-settings.json`;
+    // copy through the per-agent runtime dir so claude reads it from the
+    // same socket-adjacent location every time and so a future override
+    // (per-agent settings JSON layer) drops in cleanly.
+    let src = hive_sh4re::assets::claude_settings();
+    tokio::fs::copy(&src, &path)
+        .await
+        .with_context(|| format!("copy claude settings from {} to {}", src.display(), path.display()))?;
     tracing::info!(path = %path.display(), "wrote claude settings");
     Ok(path)
 }
@@ -508,10 +514,10 @@ pub async fn wait_for_login(
 /// regular files + newest `mtime` across them. The two axes are both
 /// load-bearing for `wait_for_login`'s refresh check (`session_refreshed`):
 /// mtime catches the common case (re-login overwrites an existing
-/// credentials file in-place), file_count catches the pathological case
+/// credentials file in-place), `file_count` catches the pathological case
 /// where `meta.modified()` errors on every file (exotic fs, NFS quirks)
 /// so the mtime axis stays `None` forever but new files still trigger a
-/// resume. Defaults to `{0, None}` on read_dir failure (missing or
+/// resume. Defaults to `{0, None}` on `read_dir` failure (missing or
 /// unreadable dir) — `wait_for_login` then resumes when files first
 /// appear.
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
@@ -542,9 +548,9 @@ fn snapshot_dir(dir: &Path) -> DirSnapshot {
 /// Has the credentials dir been written since `prev`? Used as the
 /// exit condition for `wait_for_login`:
 ///
-/// - file_count changed → something was added or removed, treat as
+/// - `file_count` changed → something was added or removed, treat as
 ///   refresh (covers the "all files have unreadable mtime" edge case).
-/// - newest_mtime advanced → existing file was rewritten in place
+/// - `newest_mtime` advanced → existing file was rewritten in place
 ///   (the common claude re-login path).
 /// - prev had no mtime (empty or all-unreadable) and now has one →
 ///   first useful signal we've seen, treat as refresh.