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/otel_turn_metrics.rs

@@ -53,7 +53,7 @@ struct Instruments {
     open_threads: Gauge<u64>,
     open_reminders: Gauge<u64>,
     /// Recorded from [`crate::claude_md_watch`]'s own tick, not from
-    /// [`record`] — `CLAUDE.md` size is a continuously-live value (like
+    /// [`record`] — `AGENTS.md` size is a continuously-live value (like
     /// hive-c0re's container gauges), not a once-per-turn one, so it has
     /// its own entry point rather than riding `record`'s per-turn call.
     claude_md_lines: Gauge<u64>,
@@ -110,7 +110,7 @@ pub fn record(row: &TurnStatRow, fresh_session: bool) {
     }
 }
 
-/// Record the current `CLAUDE.md` line count. Called from
+/// Record the current `AGENTS.md` line count. Called from
 /// [`crate::claude_md_watch`]'s periodic tick (every ~15 minutes, not
 /// per turn) — see the field doc on [`Instruments::claude_md_lines`] for
 /// why this doesn't ride [`record`]. No-op when OTEL isn't configured,