feat(CUST-WP-0015): implement agent inbox for inter-agent coordination

Adds a message-passing layer to state-hub so Claude instances can
coordinate across sessions without polling shared progress events.

- Migration f3a4b5c6d7e8: agent_messages table with thread support
- FastAPI router: POST/GET /messages/, thread view, mark-read, archive, reply
- 4 MCP tools: send_message, get_messages, mark_message_read, reply_to_message
- Observable dashboard: /inbox page with unread/read/archived sections + KPI
- CLAUDE.md updates: global, custodian, marki-docx, activity-core, template
- TOOLS.md: Agent Inbox tools section documented

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/main.py

@@ -5,7 +5,7 @@ from fastapi.middleware.cors import CORSMiddleware
 
 from api.database import engine
 from api.routers import decisions, extension_points, progress, state, tasks, technical_debt, topics, workstreams, workstream_dependencies
-from api.routers import domains, repos, contributions, sbom, policy, domain_goals, repo_goals
+from api.routers import domains, repos, contributions, sbom, policy, domain_goals, repo_goals, messages
 
 
 @asynccontextmanager
@@ -42,6 +42,7 @@ app.include_router(domain_goals.router)
 app.include_router(repo_goals.router)
 app.include_router(contributions.router)
 app.include_router(sbom.router)
+app.include_router(messages.router)
 app.include_router(state.router)
 app.include_router(policy.router)