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_summary_cache.py

@@ -142,7 +142,7 @@ def test_summary_cache_unit_progress_section():
         blocking_decisions=[],
         waiting_tasks=[],
         recent_progress=[],
-        open_workstreams=[],
+        open_workplans=[],
     )
     cache.store(summary, rev)
 
@@ -181,7 +181,7 @@ def test_invalidate_summary_cache_scopes():
         blocking_decisions=[],
         waiting_tasks=[],
         recent_progress=[],
-        open_workstreams=[],
+        open_workplans=[],
     )
     cache.store(summary, rev)