feat: Add /PACT:cleanup-worktrees automation command

[michael-wojcik]

Summary

Add a command to automate worktree detection and cleanup.

Context

Currently orphaned worktrees require manual git worktree prune. A dedicated command could automate this.

Proposed Implementation

1. Create /PACT:cleanup-worktrees command
2. Detect orphaned worktrees via git worktree list
3. Prompt user for confirmation before cleanup
4. Execute cleanup protocol for each

Related

• Phase E Worktree Integration (PR feat: Worktree Integration (Phase E) #144)
• Issue feat: Add session cleanup hooks for orphaned worktrees #145 (Session cleanup hooks)
• pact-worktree.md Abort/Crash Recovery section