feat(agents): manifest plumbing + GET /agents/registry (ADR-0014 step 3)

Each agent now exports a module-level MANIFEST declaring id, version,
pref_schema, required_consents, ttl_sec, and silenced_in_contexts. The
registry surfaces both the agent and its manifest, and rejects on
mismatch so the two cannot drift.

ml/serving exposes GET /agents/registry; services/api proxies it as
GET /api/agents/registry with a 60s in-process cache so admin pageviews
don't hammer upstream. Failures aren't cached.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ml/serving/tests/test_registry_endpoint.py

@@ -0,0 +1,21 @@
+"""GET /agents/registry — manifests are exposed in JSON-serialisable form."""
+import pytest
+from httpx import AsyncClient, ASGITransport
+
+from main import app
+
+
+@pytest.mark.anyio
+async def test_registry_returns_all_agents():
+    transport = ASGITransport(app=app)
+    async with AsyncClient(transport=transport, base_url="http://test") as client:
+        resp = await client.get("/agents/registry")
+
+    assert resp.status_code == 200
+    payload = resp.json()
+    ids = {a["id"] for a in payload["agents"]}
+    assert ids == {"overdue-task", "momentum", "time-of-day", "recent-patterns", "focus-area"}
+
+    sample = payload["agents"][0]
+    for key in ("id", "version", "description", "pref_schema", "required_consents", "ttl_sec"):
+        assert key in sample