Agent Skills: 1k-worktree-clean

Use when the user wants to audit local git worktrees against origin/x, choose by number or A, and decide whether stale worktree directories, already-merged worktrees, or no-delta worktrees can be cleaned up. Triggers on "worktree clean", "worktree 合并到 x", "检测哪些 worktree 已进 x".