STATE-WP-0070 T04 (partial): drop open_workstreams from /state/summary

Removes the redundant open_workstreams mirror field from StateSummary (schema,
router, MCP get_domain_summary reader, dashboard consumers + empty-state stub,
and tests). Consumers already preferred open_workplans, so this is the
low-risk half of T04.

Deferred (still have live callers — not yet retirement-ready):
- workstream_id query/body field alias on preferred routes — external
  session-close curls/scripts fleet-wide still send it.
- flows/workstream.yaml — /flows/workstream/{id} routes are still served and
  exercised by tests; retire only once no callers remain.

Staged on branch state-wp-0070-legacy-retirement — do not merge until the 7th
documented zero-usage window is captured.

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

tests/test_mcp_smoke.py

@@ -48,14 +48,14 @@ class TestGetStateSummary:
         assert r.status_code == 200
         body = r.json()
         # Required top-level fields
-        for key in ("open_workstreams", "blocking_decisions", "blocked_tasks",
+        for key in ("open_workplans", "blocking_decisions", "blocked_tasks",
                     "domains", "contribution_counts", "licence_risk_count"):
             assert key in body, f"missing key: {key}"
 
     async def test_empty_db_returns_zero_counts(self, client):
         r = await client.get("/state/summary")
         body = r.json()
-        assert body["open_workstreams"] == []
+        assert body["open_workplans"] == []
         assert body["blocking_decisions"] == []
         assert body["blocked_tasks"] == []