Rename backfill-status to fix-up as the general migration entry point
backfill-status was a one-off; replace it with tsk fix-up that runs
every known one-shot migration in sequence. Today that's just the
status backfill; future migrations land in the same handler. Each
migration must stay idempotent so re-running fix-up is safe. AGENTS.md
documents the new command.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>