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

claude-plugins/plugins/base/skills/claude-subagents/SKILL.md

@@ -37,7 +37,7 @@ actually attached, so a `continue` naming a session that isn't there
 fails the tool call outright instead of looking like it worked; the error
 says which directory it searched, which is usually the fix (pass `dir`).
 `dir` defaults to your work dir, `/agents/<you>/work`, which has no
-`CLAUDE.md` above it, so a subagent started there does not load yours
+`AGENTS.md` above it, so a subagent started there does not load yours
 (until that directory exists, the daemon's cwd is used).
 Use `status` for a
 zero-cost "is it still going" check — for a running subagent it also

claude-plugins/plugins/base/skills/state-hygiene/SKILL.md

@@ -5,7 +5,7 @@ description: Enforces safe handling of an agent's own durable state/notes files
 
 # State Hygiene
 
-Durable notes/state files (your `CLAUDE.md`, `state/notes.md`, `state/TODO.md`,
+Durable notes/state files (your `AGENTS.md`, `state/notes.md`, `state/TODO.md`,
 and anything else under `state/`) are your memory across container resets and
 context compactions. They only work if you actually read them before you
 change them - writing blind is how history gets silently destroyed or