Add tsk backfill-status for tasks created before auto-status
Walks the active namespace's mapping and sets status=open on any task
whose tree carries no status property yet. Tasks already marked done
are skipped, so re-running is a no-op.
Tests: workspace::backfill_status_marks_legacy_tasks_open_and_skips_done
covers the legacy + fresh-open + dropped mix and the idempotency
property.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>