rename claude.md to agents.md for agents and the repo

Claude Code (>= 2.1.277) and opencode both read AGENTS.md, but Claude
Code reads it only when no CLAUDE.md exists in its cwd or above. So
this is a plain rename with no CLAUDE.md stub left behind.

- hive-agent/src/agents_md_migrate.rs: new. At harness start, before
  the first turn, renames state/CLAUDE.md to state/AGENTS.md when only
  the former exists; logs an error naming both paths when both exist
  and touches neither; does nothing otherwise. Idempotent.
- hive-agent/src/main.rs: calls it in serve_main; checkpoint prompt
  and a comment name AGENTS.md.
- hive-agent/src/claude_md_watch.rs: the size watch reads AGENTS.md;
  todo text, doc comments and tests follow. Module name, todo
  subsystem key ("claude_md") and metric name
  (hyperhive.agent.claude_md.lines) are unchanged so existing todo
  rows and dashboard history carry over.
- hive-agent/src/turn.rs, hive-agent/prompts/system.md: prompts tell
  the agent its file is AGENTS.md.
- hive-agent/src/{otel_turn_metrics,stats}.rs,
  frontend/packages/agent/src/stats.js, hive-subagent-mcp/src/*.rs:
  comments and a tool parameter description naming the agent's file.
- nix/host-modules/swarm-grafana/dashboards/agents.json: panel title
  and description.
- claude-plugins skills claude-subagents, state-hygiene: they mean the
  agent's own state file.
- CLAUDE.md -> AGENTS.md at the repo root; docs/crates/README.md link
  (checked by check-doc-refs.sh), hive-c0re/README.md,
  .prettierignore and the nix/sources.nix comment follow.
- docs/: every prose mention of the agent's file; persistence.md
  documents the boot-time rename, claude-invocation.md the shadowing.

Fixes #5509

hive-agent/src/claude_md_watch.rs

@@ -1,5 +1,5 @@
-//! `CLAUDE.md` size watch: raises a loose-ends-v2 todo when the agent's
-//! own `CLAUDE.md` grows past 500 lines, and feeds the same line count
+//! `AGENTS.md` size watch: raises a loose-ends-v2 todo when the agent's
+//! own `AGENTS.md` grows past 500 lines, and feeds the same line count
 //! into an OTEL gauge every tick regardless of threshold. Per the
 //! operator's own framing of the ask — "claude md is personality and
 //! how you organize yourself, not a work log" — the prompt is asked to
@@ -11,7 +11,7 @@
 //! summary) — a third instance of the same anti-nag pattern rather than
 //! a new one.
 //!
-//! `CLAUDE.md` lives at the top of the agent's own state dir — the same
+//! `AGENTS.md` lives at the top of the agent's own state dir — the same
 //! directory `claude --print` runs with as its cwd (`turn::session_cwd`),
 //! which is how the CLI's own auto-load discovers it.
 //!
@@ -40,20 +40,24 @@ const WARN_LINES: u64 = 500;
 const BUCKET: u64 = 50;
 /// Producing subsystem marker for the todo row.
 const SUBSYSTEM: &str = "claude_md";
-/// Fixed dedup key — there is only ever one `CLAUDE.md`-size todo per
+/// Fixed dedup key — there is only ever one `AGENTS.md`-size todo per
 /// agent.
 const TODO_KEY: &str = "size";
 
-/// Background loop: periodically reconcile the `CLAUDE.md`-size todo and
+/// Background loop: periodically reconcile the `AGENTS.md`-size todo and
 /// feed the OTEL gauge. Detached task — runs for the harness's lifetime;
 /// errors are logged, never fatal.
 pub async fn run(todos: Arc<Todos>, wake: Arc<Notify>) {
     loop {
         tokio::time::sleep(CHECK_INTERVAL).await;
-        let lines = match tokio::task::spawn_blocking(|| line_count(&claude_md_path())).await {
+        let lines = match tokio::task::spawn_blocking(|| {
+            line_count(&agents_md_path(&crate::paths::state_dir()))
+        })
+        .await
+        {
             Ok(l) => l,
             Err(e) => {
-                tracing::warn!(error = %e, "claude.md watch probe panicked");
+                tracing::warn!(error = %e, "agents.md watch probe panicked");
                 continue;
             }
         };
@@ -72,25 +76,25 @@ fn reconcile(todos: &Todos, wake: &Notify, summary: Option<&str>) {
         Some(summary) => match todos.upsert(SUBSYSTEM, Some(TODO_KEY), summary, None, false) {
             Ok((_, true)) => wake.notify_one(),
             Ok((_, false)) => {}
-            Err(e) => tracing::warn!(error = ?e, "claude.md todo upsert failed"),
+            Err(e) => tracing::warn!(error = ?e, "agents.md todo upsert failed"),
         },
         None => {
             if let Err(e) = todos.clear(SUBSYSTEM, Some(TODO_KEY)) {
-                tracing::warn!(error = ?e, "claude.md todo clear failed");
+                tracing::warn!(error = ?e, "agents.md todo clear failed");
             }
         }
     }
 }
 
-/// `CLAUDE.md`'s path: the top of the agent's own state dir, matching
+/// `AGENTS.md`'s path: the top of the agent's own state dir, matching
 /// where `claude --print` runs (`turn::session_cwd`) and therefore where
 /// its own auto-load discovers the file.
-fn claude_md_path() -> std::path::PathBuf {
-    crate::paths::state_dir().join("CLAUDE.md")
+fn agents_md_path(state_dir: &Path) -> std::path::PathBuf {
+    state_dir.join("AGENTS.md")
 }
 
 /// Line count of the file at `path`. `None` when it can't be read — no
-/// `CLAUDE.md` yet is not a warning, and a transient read failure isn't
+/// `AGENTS.md` yet is not a warning, and a transient read failure isn't
 /// either.
 fn line_count(path: &Path) -> Option<u64> {
     let contents = std::fs::read_to_string(path).ok()?;
@@ -108,9 +112,9 @@ fn summary_for(lines: u64) -> Option<String> {
     }
     let bucket = lines / BUCKET * BUCKET;
     Some(format!(
-        "your CLAUDE.md is {bucket}+ lines. CLAUDE.md is personality and how you organize \
+        "your AGENTS.md is {bucket}+ lines. AGENTS.md is personality and how you organize \
          yourself, not a work log — if it's grown past that, moving detail into separate files \
-         (linked from CLAUDE.md) or an archive is what actually shrinks this count. It \
+         (linked from AGENTS.md) or an archive is what actually shrinks this count. It \
          auto-loads into every turn, so a smaller file is a fixed cost saved on every turn. \
          Nothing here is a hard limit, and deleting stale content outright is fine too if \
          that's genuinely what it is — no obligation either way. No need to act on this now — \
@@ -175,16 +179,28 @@ mod tests {
     #[test]
     fn line_count_matches_editor_expectations() {
         let dir = tempfile::tempdir().expect("tempdir");
-        let path = dir.path().join("CLAUDE.md");
+        let path = dir.path().join("AGENTS.md");
         // Three lines, trailing newline — an editor shows 3 lines, not 4.
         std::fs::write(&path, "one\ntwo\nthree\n").expect("write");
         assert_eq!(line_count(&path), Some(3));
     }
 
+    /// The watch counts `AGENTS.md`, and a leftover `CLAUDE.md` beside it
+    /// is not what it reads.
+    #[test]
+    fn watch_reads_agents_md_not_claude_md() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        std::fs::write(dir.path().join("CLAUDE.md"), "one\n").expect("write");
+        assert_eq!(line_count(&agents_md_path(dir.path())), None);
+
+        std::fs::write(dir.path().join("AGENTS.md"), "one\ntwo\n").expect("write");
+        assert_eq!(line_count(&agents_md_path(dir.path())), Some(2));
+    }
+
     #[test]
     fn missing_file_is_silent_not_an_error() {
         assert_eq!(
-            line_count(Path::new("/definitely/does/not/exist/CLAUDE.md")),
+            line_count(Path::new("/definitely/does/not/exist/AGENTS.md")),
             None
         );
     }