fix: unblock_task returns to todo and clears stale claims

[danielkov]

Summary

• unblock_task now always transitions to todo instead of checking started_at to decide between in_progress/todo
• Clears stale claim fields (claim_owner, claim_claimed_at, claim_lease_expires_at) on unblock so the task.next SQLite trigger fires correctly
• Fixes a bug where unblocked tasks were invisible to worker daemons because no task.next event was emitted

Root cause

unblock_task checked started_at.is_some() to decide the target status. Since agents set started_at when they begin work, unblocking always returned to in_progress. Additionally, stale claim fields were preserved, so even manually cycling through draft → todo couldn't trigger task.next — the trigger's claim guard suppressed it:

AND (NEW.claim_owner IS NULL OR NEW.claim_lease_expires_at < strftime(...))

✻ Clauded...