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/observablehq.config.js

@@ -27,6 +27,7 @@ export default {
     { name: "Contributions", path: "/contributions" },
     { name: "SBOM",          path: "/sbom" },
     { name: "Repo Sync",     path: "/repo-sync" },
+    { name: "Inbox",         path: "/inbox" },
     { name: "Progress",      path: "/progress" },
     // ── Policy ────────────────────────────────────────────────────────────────
     {