source-control/babysit-prs: harden prune_babysit_worktrees.py against orphaned state

[kyle-sexton]

This was generated by AI during triage.

Two robustness gaps in prune_babysit_worktrees.py observed at queue-start prune:

• A leftover state record for a merged/closed PR whose worktree directory no longer exists makes prune error fatal: not a git repository on every run instead of self-healing. When git status (or equivalent) fails with "not a git repository" for a tracked worktree path, treat that path as already-gone and drop its state record.
• On Windows a file lock blocking removal can leave an empty directory behind, requiring manual rm -rf + git worktree prune. Detect the residual empty directory and either retry/clean it or report it explicitly rather than leaving a silent orphan.

Acceptance: an orphaned state record pointing at a nonexistent path is dropped without erroring the whole prune; a lock-blocked removal does not silently leave an untracked empty directory (it is cleaned or clearly reported).

Sweep-converted from #657.

Work-class: C3 (bug-fix-shaped) — attended triage 2026-07-23, operator-ratified. 🤖

[kyle-sexton]

This was generated by AI on operator direction.

Superseded — this item was already ratified and should not have been queued.

The earlier first-drain ratification request on this issue was raised in error. The admission gate dispatches a C3 bug-fix-shaped item when it carries either its own recorded ratification or a completed first drain — and this issue already carried recorded ratification in its body from attended triage on 2026-07-23. The drain cycle applied the blanket first-drain rule without first reading bodies for an existing ratification, so it queued an item that was already admitted.

Restored to the autonomous frontier: agent-ready re-applied, needs-human removed. No human action is required, and this issue no longer appears in the attended queue's ratify view.

[kyle-sexton]

This was generated by AI during the work-loop admission gate.

Classification

• Work class: C3 scoped, bug-fix-shaped — prune_babysit_worktrees.py currently assumes
  well-formed worktree state and has no defensive handling for orphaned entries; the fix is a
  scoped hardening of existing logic, not a new capability or contract change.
• Type: Bug (matches the tracker's existing Bug issue type on this item).

Intended dispatch

Queued for the first-drain C3 ratification gate per the loop-lane convention's earn-trust rule —
first_drain_complete is still false in this repository's work-loop durable state, so this
bug-fix-shaped C3 candidate cannot dispatch until it carries its own recorded ratification. Once
/work-items:attend-queue replies with ratification on this comment (or first_drain_complete
flips true on a completed drain), it returns to the autonomous frontier and dispatches on a later
work-loop cycle — no re-queue needed.