skill: /tidy-worktrees — audit and remove worktrees whose PRs have merged

Problem

Nothing closes the loop after a PR merges. /start-work creates a worktree and branch; /craft-pr is described as its bookend but ships the work rather than cleaning up after it. The result is unbounded accumulation.

Measured in TimZander/AudioClassifier on 2026-08-05, right after merging a PR:

12 worktrees:  10 MERGED PRs,  1 CLOSED PR,  1 OPEN PR   (55 MB)

11 of 12 stale. All clean — no uncommitted work anywhere — so cleanup was safe the whole time and simply never happened.

Why a skill rather than documentation

Companion issue #214 adds the rule to standards/CLAUDE.md. This issue is the execution, and they are not substitutes:

1. The decision needs a gh fan-out that docs cannot perform. Under squash-merge git branch --merged reports every merged branch as unmerged (see #214), so the only reliable signal is one gh pr list --head <branch> call per worktree. That is a multi-step tool sequence, which is the skill trigger.
2. A documented manual procedure demonstrably does not get run. The repo above documents worktree discipline carefully — mandating worktrees, giving the exact git worktree add invocation, warning about nesting — and still reached 11 stale worktrees.
3. The safety conditions are easy to get wrong by hand. Never remove the current worktree; never remove the primary (in that repo the primary must stay on main or the pinned core.hooksPath stops resolving); never remove a dirty tree; use -D deliberately rather than habitually.

Proposed behaviour

/tidy-worktrees [--yes] [--include-closed]

Default is a dry-run report — never destructive without confirmation.

1. Enumerate via git worktree list --porcelain, resolving each to its branch.
2. Skip the primary checkout and the current worktree, always, and say so in the output.
3. For each remaining worktree, resolve PR state: gh pr list --head <branch> --state all --json number,state.
4. Check git -C <worktree> status --porcelain; anything dirty is excluded and reported, never force-removed.
5. Classify:
   • MERGED → proposed for removal
   • CLOSED → listed but not proposed unless --include-closed (abandoned work may still be wanted)
   • OPEN / no PR → listed as active, never proposed
6. Print a table, confirm via AskUserQuestion, then per approved entry: git worktree remove → git branch -D → git worktree prune once at the end.

Acceptance criteria

• Dry-run by default; no destructive command runs before explicit confirmation
• Correctly identifies squash-merged branches that git branch --merged misses
• Refuses to touch the current worktree, the primary checkout, or any dirty worktree — each refusal reported with its reason
• CLOSED-PR and no-PR worktrees are never removed by default
• git worktree prune runs once after removals, not per-iteration
• Output names each worktree, its branch, its PR number and state, and the action taken or why it was skipped
• Handles the no-gh/unauthenticated case by reporting rather than falling back to a guess
• Works from inside a worktree (the common case — you have just merged and are still standing in it)

Non-goals

• Deleting remote branches. Hosts auto-delete on merge, and this should not race that.
• Anything touching a branch with no merged PR. Discovering abandoned work is a different job.

Start by reading the existing /start-work and /craft-pr entry points and the worktree guidance in standards/CLAUDE.md. Trace how a new /tidy-worktrees skill can use git worktree list --porcelain, git status, and gh pr list. Done means the acceptance criteria are met, including dry-run confirmation, safety skips, PR-state reporting, and one final prune.