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>

dashboard/src/data/messages.json.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+"""Observable data loader: fetches /messages/ from the API."""
+import json
+import os
+import urllib.request
+import urllib.error
+
+API_BASE = os.environ.get("API_BASE", "http://127.0.0.1:8000").rstrip("/")
+
+try:
+    with urllib.request.urlopen(f"{API_BASE}/messages/?limit=100", timeout=10) as resp:
+        data = json.loads(resp.read())
+    print(json.dumps(data))
+except urllib.error.URLError as e:
+    print(json.dumps({"error": str(e), "messages": []}))