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

nix/host-modules/swarm-grafana/dashboards/agents.json

@@ -853,8 +853,8 @@
         "kind": "Panel",
         "spec": {
           "id": 33,
-          "title": "CLAUDE.md size by agent",
-          "description": "Lines in each agent's CLAUDE.md file over time.",
+          "title": "AGENTS.md size by agent",
+          "description": "Lines in each agent's AGENTS.md file over time.",
           "links": [],
           "data": {
             "kind": "QueryGroup",

nix/sources.nix

@@ -10,7 +10,7 @@
   # does NOT change the store path and therefore does NOT force a
   # rebuild of every agent container.
   #
-  # Dropped: scripts/, docs/, root-level *.md (README/CLAUDE/TODO/…).
+  # Dropped: scripts/, docs/, root-level *.md (README/AGENTS/TODO/…).
   # Kept (build needs them): .nix, .rs, Cargo.*, branding/,
   # frontend/, prompts/, flake.lock. docs/ is deliberately dropped:
   # it is shipped to agent containers as its OWN narrow meta-flake