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/main.rs

@@ -9,6 +9,7 @@
 //! Single bin crate: the module tree below (formerly this crate's `lib.rs`,
 //! before lib + bin were collapsed into one) plus the serve loop.
 
+mod agents_md_migrate;
 mod boot_announce;
 mod claude_credentials;
 mod claude_md_watch;
@@ -215,7 +216,7 @@ fn graceful_stop_message() -> hive_sh4re::inbox::DeliveredMessage {
         body: "You are being gracefully stopped — the container will shut down after this turn, \
                and new inbound messages are already fenced. This is your one checkpoint turn: \
                flush anything worth keeping into your durable /state files now — update your \
-               notes / CLAUDE.md / TODO.md with in-flight task state, decisions made, important \
+               notes / AGENTS.md / TODO.md with in-flight task state, decisions made, important \
                file paths, and whatever you'd need to resume cleanly later with only a summary \
                of this conversation to go on. Do not start new work or reply to anyone; just \
                write your notes and end your turn. The session may be compacted after this turn \
@@ -450,7 +451,7 @@ fn spawn_todo_socket(
             // top level instead of disk pressure.
             tokio::spawn(state_entry_watch::run(store.clone(), todo_wake.clone()));
             // Same shape again: nudge (and feed an OTEL gauge) on a
-            // grown CLAUDE.md instead of a grown state dir.
+            // grown AGENTS.md instead of a grown state dir.
             tokio::spawn(claude_md_watch::run(store.clone(), todo_wake.clone()));
             // Swarm-scheduled prompts fire as todos too, from the list the
             // swarm keeps for this agent.
@@ -553,6 +554,8 @@ async fn serve_main<S: Surface>(socket: &Path, poll_ms: u64) -> Result<()> {
             tracing::error!(error = %e, "web_ui::serve exited with error");
         }
     });
+    // Before the first turn, so claude's own auto-load finds `AGENTS.md`.
+    agents_md_migrate::run(&paths::state_dir());
     // Harness-local reminders: a due reminder fires
     // straight into an mpsc channel the serve loop races against the
     // broker long-poll (unlike todos, a fire carries real per-row data,