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

docs/agent-lifecycle/persistence.md

@@ -270,6 +270,13 @@ Under `/var/lib/hyperhive/agents/<name>/`:
   - `hyperhive-harness.json` — rate-limited / needs-login flags read
     by the dashboard's async container-state fetch. See
     `docs/web-ui/dashboard.md::Container row`.
+
+  The agent writes its own instructions to `AGENTS.md` here; claude and
+  opencode load it automatically from their cwd, `state/`. At boot,
+  `agents_md_migrate::run` renames a `state/CLAUDE.md` to `AGENTS.md`
+  when no `AGENTS.md` exists. When both exist it renames neither and
+  logs an error naming both paths: claude loads only `CLAUDE.md` while
+  one exists in its cwd or above.
 - `harness/` — harness-internal ephemeral state; not intended for
   agent consumption. Bind-mounted to `/agents/<name>/harness`
   inside the container (`$HYPERHIVE_HARNESS_DIR`). Contents:
@@ -321,7 +328,7 @@ Under `/var/lib/hyperhive/agents/<name>/`:
   and other large working data. Bind-mounted RW to
   `/agents/<name>/work` inside the container and owned by the agent
   user. It sits outside `state/` so a claude or subagent started under
-  `work/` doesn't walk up into `state/CLAUDE.md` and load the agent's
+  `work/` doesn't walk up into `state/AGENTS.md` and load the agent's
   own identity. Survives `destroy`/recreate, gone on `--purge`. Counted
   in the per-agent disk metric (`measure_agent_disk`) together with
   `state/` and the container rootfs. The dashboard doesn't serve it: