tools/office/reap.py: reap sessions that were archived and so never wrapped up nf-8tw4 ← Beads

Twinned from https://github.com/cfsh/ninefold/issues/641 by tools/beads/import_github.py's reconcile pass.

Aedan archives sessions from the Claude UI when an agent is done, which kills nothing on the box — the tmux session, the process and both worktrees survive, and an archived session cannot run its own wrap-up (#640). So the CTO reaps by hand; today that was four sessions. Option B of [cfsh/nf-office#3](https://github.com/cfsh/nf-office/issues/3), agreed by @aedanpope.

A session is provably reapable when its PRs are merged/closed, its claimed issues are closed or RELEASEd, and tracked-prs.txt is empty — all three, not just the first. claude agents --json gives what is live.

⚠️ Must refuse argo and bork by name; they are @aedanpope's standing slots and are never reaped, whatever the checks say. Shares its reaping logic with #640 — one implementation, two entry points. Overlaps #635.