STATE-WP-0069 T04/T06: body metering and workplan-first state internals

Share LegacyWorkstreamIdBodyMixin across create schemas; meter POST /tasks/
and /decisions/ workstream_id bodies. State summary uses workplan flow;
NextStep dual-writes workplan_* fields alongside legacy workstream_*.

api/schemas/state.py

@@ -64,6 +64,9 @@ class NextStep(BaseModel):
     """
     type: str          # unblocked_task | resolved_decision | dependency_cleared
     domain: str | None = None
+    workplan_id: uuid.UUID | None = None
+    workplan_title: str | None = None
+    workplan_slug: str | None = None
     workstream_id: uuid.UUID | None = None
     workstream_title: str | None = None
     workstream_slug: str | None = None