Add hub-core package, docs, and State Hub integration scaffold

Extract the first reusable slice (models, schemas, routers, MCP, migrations)
from state-hub with INTENT/SCOPE, agent instructions, workplan, and aligned
inter_hub capability registry index.

.custodian-brief.md

@@ -0,0 +1,27 @@
+<!-- custodian-brief: generated by statehub register; fix-consistency may replace this file -->
+# Custodian Brief - hub-core
+
+**Project:** hub-core
+**Domain:** inter_hub
+**State Hub:** http://127.0.0.1:8000
+**Topic ID:** `1f2e4d10-c967-4803-ae6c-7f4b4e806409`
+
+## Open Workplans
+
+### Bootstrap State Hub integration
+
+Workplan file: `workplans/HUB-WP-0001-statehub-bootstrap.md`
+
+Open tasks:
+- T01 - Review generated integration files
+- T02 - Verify local developer workflow
+- T03 - Seed first real workplan
+
+## Session Start
+
+1. Read `INTENT.md`, `SCOPE.md`, and `AGENTS.md`.
+2. Check inbox: `GET /messages/?to_agent=hub-core&unread_only=true`.
+3. Scan `workplans/`.
+4. Update task statuses in workplan files as work progresses.
+
+Last generated: 2026-06-16