fix(openclaw-tps-mail): a terminal obligation whose cur/ record is gone counts as resolved for retention

[tps-flint]

Found while verifying the post-merge audit of #500 (medium).

Startup reconciliation treats a terminal obligation whose cur/ record no longer exists (archived or garbage-collected) as resolved, so ordinary retention applies to it and no repeated restart warning is logged for it.

Acceptance: a test where the cur/ record has been archived shows the obligation swept at its retention age.