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>

api/schemas/state.py

@@ -83,7 +83,6 @@ class StateSummary(BaseModel):
     waiting_tasks: list[TaskRead]
     blocked_tasks: list[TaskRead] = []
     recent_progress: list[ProgressEventRead]
-    open_workstreams: list[WorkstreamWithDeps]
     open_workplans: list[WorkstreamWithDeps] = []
     next_steps: list[NextStep] = []
     domains: list[DomainSummary] = []