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>

CLAUDE.md

@@ -88,7 +88,8 @@ Every Claude Code session in this repository must follow this ritual:
 
 **On session start:**
 1. Call `get_state_summary()` via the `state-hub` MCP tool for orientation
-2. Note any blocking decisions or blocked tasks before starting work
+2. Check the agent inbox: `get_messages(to_agent="hub", unread_only=True)` — mark read and act on any messages
+3. Note any blocking decisions or blocked tasks before starting work
 
 **On session close (before ending):**
 1. Call `add_progress_event()` to log what was done, decided, or discovered