Stop dual-writing workstream_id on State Hub progress POSTs

Progress scope helpers and schedule-miss alerts now send workplan_id only.
Legacy workstream_id in sink config still maps to workplan_id on the wire.

src/activity_core/schedule_health.py

@@ -178,7 +178,6 @@ def post_missed_fire_alert(
     scope_id = workplan_id or workstream_id
     if scope_id:
         body["workplan_id"] = scope_id
-        body["workstream_id"] = scope_id
 
     # Dedup repeated alerts for the same missed window (same schedule + last fire).
     last_fired = health.last_fired_at.isoformat() if health.last_fired_at else "none"

src/activity_core/state_hub_write.py

@@ -23,13 +23,12 @@ IDEMPOTENCY_HEADER = "Idempotency-Key"
 def apply_progress_scope_fields(body: dict[str, Any], source: dict[str, Any]) -> None:
     """Copy topic/workplan/task/decision scope from a sink config into a progress body.
 
-    Prefers ``workplan_id``; dual-writes ``workstream_id`` for wire-compat until
-    legacy-meter retires the alias.
+    Prefers ``workplan_id``; accepts legacy ``workstream_id`` in sink config only
+    as a fallback source (maps to ``workplan_id`` on the wire).
     """
     workplan_id = source.get("workplan_id") or source.get("workstream_id")
     if workplan_id:
         body["workplan_id"] = workplan_id
-        body["workstream_id"] = workplan_id
     for key in ("topic_id", "task_id", "decision_id"):
         if source.get(key):
             body[key] = source[key]