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/schemas/agent_message.py

@@ -0,0 +1,30 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict
+
+
+class MessageCreate(BaseModel):
+    from_agent: str
+    to_agent: str
+    subject: str
+    body: str
+    thread_id: uuid.UUID | None = None
+
+
+class MessageReply(BaseModel):
+    from_agent: str
+    body: str
+
+
+class MessageRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+    id: uuid.UUID
+    from_agent: str
+    to_agent: str
+    subject: str
+    body: str
+    thread_id: uuid.UUID | None = None
+    read_at: datetime | None = None
+    archived_at: datetime | None = None
+    created_at: datetime