Feature: Delete associated worktrees when removing stale branches

[Sixeight]

Description

Summary

Add worktree support to gh-poi: when a branch is identified as deletable, automatically remove any associated worktree before deleting the branch.

Motivation

Git worktrees allow developers to have multiple working directories from a single repository. A common workflow is:

git worktree add ../feature-123 feature-123
# work on feature, create PR, merge
gh poi  # currently fails or leaves orphaned worktree

Currently, gh poi cannot delete branches that have associated worktrees because:

1. Git refuses to delete a branch checked out in any worktree
2. After manual branch deletion, orphaned worktrees remain on disk

This forces users to manually manage worktrees, defeating the purpose of automated cleanup.

Proposed Solution

Behavior

1. Detection: Before attempting branch deletion, check if the branch has an associated worktree using git worktree list
2. Removal: For deletable branches with worktrees, run git worktree remove <path> before git branch -D
3. Display: Show worktree paths in the output when they will be removed

New Flag

Flag	Description
--worktree	Also remove worktrees associated with deletable branches

Output Example

$ gh poi --worktree

Deleting branches...
  ✓ feature-123 (worktree: /home/user/projects/feature-123)
  ✓ bugfix-456 (worktree: /home/user/projects/bugfix-456)
  ✓ hotfix-789

3 branches deleted (2 worktrees removed)

Dry-run Support

$ gh poi --dry-run --worktree

Branches to delete:
  - feature-123 (worktree: /home/user/projects/feature-123)
  - bugfix-456 (worktree: /home/user/projects/bugfix-456)
  - hotfix-789

Additional context

Edge Cases

Case	Handling
Locked worktree	Skip with warning (user must run git worktree unlock manually)
Worktree has uncommitted changes	Skip with warning (user must handle manually)
Main worktree (repo root)	Never remove (implicit protection)
Worktree path no longer exists	Run git worktree prune first

[seachicken]

Thanks for that good point! Since worktree is a standard git feature, it should be considered when running.
Adding a worktree cleanup function will require a slightly larger design change, so I'd like to split it into smaller issues.

gh poi # currently fails or leaves orphaned worktree

To ensure we're on the same page, could you please write down the steps to take if it fails?

Also, I thought it would be easy to delete the worktree using gh poi without any options if certain conditions are met, such as the worktree commits having already been pushed to the remote. What do you think? (If you're unsure, you can check with the --dry-run option.)