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/stats.rs

@@ -142,7 +142,7 @@ pub struct Snapshot {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub reminder_stats: Option<ReminderStats>,
     /// First-turn input tokens of the most recent fresh claude session
-    /// that started in the window — a proxy for system-prompt + CLAUDE.md
+    /// that started in the window — a proxy for system-prompt + AGENTS.md
     /// sprawl (a fresh session's first turn pays the full static prefix
     /// uncached, so this is the current "cold context" cost). `None` until
     /// the sessions capture (per-session `session_id`) has data; every
@@ -379,7 +379,7 @@ fn snapshot(path: &Path, window: Window) -> Result<Snapshot> {
 }
 
 /// First-turn input tokens of the most recent fresh claude session that
-/// started in `[from, now]`. Tracks system-prompt + CLAUDE.md sprawl:
+/// started in `[from, now]`. Tracks system-prompt + AGENTS.md sprawl:
 /// the first turn of a fresh session (`--continue` suppressed) pays the
 /// full static prefix as uncached input, so watching this over time
 /// surfaces creep. Uses the agreed per-session derive — the first turn