wrap_up.py: a finished session removes its own worktree and its -f5 sibling nf-9rd1 ← Beads

Twinned from https://github.com/cfsh/ninefold/issues/640 by tools/beads/import_github.py's reconcile pass.

Wrap-up currently leaves ~/nf/devs/<topic> and ~/nf/devs/<topic>-f5 on disk, so the box accumulates a pair per session — see #602. Option A of [cfsh/nf-office#3](https://github.com/cfsh/nf-office/issues/3), agreed by @aedanpope.

Non-obvious constraint: a session cannot remove the worktree it is standing in.

Deliberately does not cover the archived-session case — an archived session cannot run its own wrap-up. That is the sibling issue.