refactor(modules): extract agent-to-agent as registry-based module

Last extraction of Phase 3. Moves inter-agent messaging + create_agent +
destination projection into src/modules/agent-to-agent/. Core retains:

- `channel_type === 'agent'` dispatch in delivery.ts, guarded by
  hasTable('agent_destinations') + dynamic import into module.
- Channel-permission ACL in delivery.ts, guarded by hasTable, with
  inlined SQL (no module import from core).
- writeDestinations call in container-runner.ts, guarded by hasTable +
  dynamic import into module.
- createMessagingGroupAgent's destination side effect in db/messaging-groups.ts,
  guarded by hasTable. This is a documented transitional tier violation
  (core imports from optional module), analogous to src/access.ts.

Migration `004-agent-destinations.ts` renamed to `module-agent-to-agent-
destinations.ts` preserving `name: 'agent-destinations'` so existing DBs
don't re-run it.

delivery.ts: 600 → 449 lines. handleSystemAction's last switch case gone
(just registry + default log-and-drop). notifyAgent helper removed (only
create_agent used it).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/db/migrations/index.ts

@@ -3,7 +3,7 @@ import type Database from 'better-sqlite3';
 import { log } from '../../log.js';
 import { migration001 } from './001-initial.js';
 import { migration002 } from './002-chat-sdk-state.js';
-import { migration004 } from './004-agent-destinations.js';
+import { moduleAgentToAgentDestinations } from './module-agent-to-agent-destinations.js';
 import { migration008 } from './008-dropped-messages.js';
 import { migration009 } from './009-drop-pending-credentials.js';
 import { moduleApprovalsPendingApprovals } from './module-approvals-pending-approvals.js';
@@ -19,7 +19,7 @@ const migrations: Migration[] = [
   migration001,
   migration002,
   moduleApprovalsPendingApprovals,
-  migration004,
+  moduleAgentToAgentDestinations,
   moduleApprovalsTitleOptions,
   migration008,
   migration009,