Add Service Definition of Mature policy and health-route test

Establish policies/service-dom.md as the service-level companion to the repo
DoI and workstream DoD. Its load-bearing Core criterion is a cheap, side-effect
free health endpoint for availability probing — satisfied by the existing
GET /state/health (DB readiness, 200/503). Served automatically at
/policy/service-dom by the existing policy router.

Add a regression test asserting /state/health returns 200 with the expected
shape, since none existed (DoM Standard criterion #4).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_routers_core.py

@@ -1512,3 +1512,20 @@ class TestFabricGraphReadModel:
         summary = r.json()
         assert summary["schema_version"] is None
         assert summary["nodes_by_fabric"] == {}
+
+
+# ---------------------------------------------------------------------------
+# Health route — Service Definition of Mature (policies/service-dom.md), Core #1
+# ---------------------------------------------------------------------------
+
+async def test_health_route_reports_ok_when_db_reachable(client):
+    """The health endpoint is a cheap availability probe with no business logic.
+
+    It must return 200 and a small JSON body so agents and monitors can tell
+    "service available" from "request wrong" without side effects.
+    """
+    r = await client.get("/state/health")
+    assert r.status_code == 200, r.text
+    body = r.json()
+    assert body["status"] == "ok"
+    assert body["db"] == "connected"