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/models/__init__.py

@@ -14,6 +14,7 @@ from api.models.technical_debt import TechnicalDebt, TDStatus
 from api.models.contribution import Contribution, ContributionType, ContributionStatus
 from api.models.sbom_snapshot import SBOMSnapshot
 from api.models.sbom_entry import SBOMEntry, Ecosystem
+from api.models.agent_message import AgentMessage
 
 __all__ = [
     "Base",
@@ -32,4 +33,5 @@ __all__ = [
     "Contribution", "ContributionType", "ContributionStatus",
     "SBOMSnapshot",
     "SBOMEntry", "Ecosystem",
+    "AgentMessage",
 ]