feat(ecosystem): hub-core library lane for three-repo stack (HUB-WP-0003)

Add slugify_or_default for core-hub parity, bump to 0.2.0, document ecosystem
position in INTENT/SCOPE/README, wire capability relations to state-hub and
core-hub, and extend utils tests. Part of CUST-WP-0057 consolidation.

docs/metadata-isolation.md

@@ -0,0 +1,38 @@
+# SQLAlchemy Metadata Isolation
+
+**Updated:** 2026-07-09  
+**Workplan:** `HUB-WP-0003-T03`
+
+---
+
+## Rule
+
+`hub_core.models.base.Base` owns metadata for **shared primitive tables only**
+(domains, managed_repos, agent_messages, progress_events, capability_*, tpsc_*).
+
+Each host runtime keeps its own declarative base for host-specific tables:
+
+| Host | Base module | Session style |
+| --- | --- | --- |
+| `state-hub` | `api.models.base.Base` | Sync |
+| `core-hub` | `core_hub.db.Base` | Async |
+
+Router factories never import host models from hub-core. Hosts inject models at
+mount time:
+
+```python
+from hub_core.routers.domains import create_domains_router
+
+app.include_router(create_domains_router(get_session, domain_model=Domain, ...))
+```
+
+---
+
+## Why separate metadata
+
+- Migration ownership stays with the host that runs the database.
+- core-hub framework tables (`hubs`, `widgets`, …) must not appear in hub-core.
+- state-hub dev-hub tables (workplans, tasks, …) must not appear in hub-core.
+- Async and sync engines can coexist without forcing one base class.
+
+See `the-custodian/docs/hub-ecosystem-architecture.md` for the full stack model.