chore(serena): untrack runtime blocker ack marker leaked by 1.1.44

The 1.1.44 release commit re-captured .serena/.flow_blocker_ack.json
before the control-plane commit machinery learned to drop .serena
paths. The root release_orchestrator changed_paths filter now prevents
any recurrence; this commit removes the leaked runtime marker from
tracked history going forward.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: rldyourmnd