feat(agents): deepen agent registry — version lifecycle, lookup by key, delete
- repository.ts: add getAgentByKey, updateAgentVersion, getPublishedVersion, deleteAgent
- routes.ts: 5 new endpoints — GET /agents/by-key/:key, GET /agents/:id/published,
POST /agents/:id/versions/:vId/publish (auto-deprecates previous),
POST /agents/:id/versions/:vId/deprecate, DELETE /agents/:id (draft only)
- routes.test.ts: 6 new tests (8 total) — publish lifecycle, deprecate guard,
key lookup, delete draft-only guard
- repository.test.ts: 1 existing test unchanged