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_*.
Add Sunset to legacy compat responses (Jun 2027 planning horizon).
Meter POST /progress/ bodies that use workstream_id; wire hub-core body
hook. Consolidate workstreams deprecation headers via legacy_compat.
Add legacy_compat helper and Deprecation headers when callers filter
/tasks, /decisions, /token-events, or /execution/launch-requests with
workstream_id. Preferred workplan_id filters are unchanged. Route removal
remains gated on legacy-meter zero-usage windows.
