Fix pm rm leaking worktree dirs on slot release

[claude]

Requested by Bradley Axen · Slack thread

Before / After (plain language, what a user sees): Before, running pm rm <project> removed the project from pm's state but left the git worktree directory under .pm/pool/repo--N on disk (and a dangling entry in git worktree list), so stale worktrees accumulated over time. After, pm rm (and the stale-state cleanup paths in pm new / pm status) fully remove the worktree from disk and from git's worktree metadata.

How: Wire up the previously-dead remove_slot_worktree helper into the shared pool::release_project path. It runs git worktree remove --force (falling back to git worktree prune) against the repo's bare clone, then force-removes the dir if git left it behind. Removal is best-effort so a manually-deleted slot doesn't make pm rm error; slot.branch is now cleared alongside slot.owner. Removed two #[allow(dead_code)] markers (pool.rs, git.rs). pm cleanup is read-only and inherits the fix transitively. Note: there is no existing test covering the pool-release path; verified via cargo test (workspace green) plus a manual smoke test (add → rm leaves the pool dir empty and git worktree list clean; a pre-deleted slot exits 0).

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Generated by Claude Code

[claude]

Superseded by #816 — same fix, recreated on a new branch with the DCO sign-off in place (avoids a force-push to rewrite history).