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_routers_core.py

@@ -433,9 +433,7 @@ class TestStateSummary:
         r = await client.get("/state/summary")
         assert r.status_code == 200
         body = r.json()
-        assert "open_workstreams" in body
         assert "open_workplans" in body
-        assert body["open_workplans"] == body["open_workstreams"]
         assert "blocking_decisions" in body
         assert "blocked_tasks" in body
         assert "domains" in body
@@ -471,7 +469,7 @@ class TestStateSummary:
         r = await client.get("/state/summary")
         assert r.status_code == 200
         body = r.json()
-        summaries = {item["id"]: item for item in body["open_workstreams"]}
+        summaries = {item["id"]: item for item in body["open_workplans"]}
 
         assert summaries[blocked_ws["id"]]["status"] == "blocked"
         assert summaries[blocked_ws["id"]]["blocked_reasons"][0]["id"] == "dependencies.all_complete"