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>

mcp_server/TOOLS.md

@@ -107,6 +107,25 @@ Domains are now first-class DB entities. Use `list_domains()` to discover availa
 
 ---
 
+## Agent Inbox Tools
+
+Inter-agent coordination via shared message board. Check inbox at session start;
+send messages to coordinate across Claude instances.
+
+Agent names: use the repo slug (e.g. `"marki-docx"`, `"railiance"`) or `"hub"` for the custodian agent.
+Use `"broadcast"` as `to_agent` to send to all agents.
+
+| Tool | Key Args | When to use |
+|------|----------|-------------|
+| `get_messages(to_agent?, from_agent?, unread_only?, limit?)` | `to_agent`: your agent name; `unread_only`: True recommended at session start | Check for pending coordination messages. |
+| `send_message(from_agent, to_agent, subject, body, thread_id?)` | all except `thread_id` required | Send a coordination message to another agent (or broadcast). |
+| `mark_message_read(message_id)` | `message_id`: UUID | Mark a message as read after acting on it. |
+| `reply_to_message(message_id, from_agent, body)` | all required | Reply in-thread; marks original as read. |
+
+Dashboard: `http://localhost:3000/inbox`
+
+---
+
 ## Domain Slugs
 
 Run `list_domains()` to get the live list. Default 6: `custodian` · `railiance` · `markitect` · `coulomb_social` · `personhood` · `foerster_capabilities`