Refuse to remove a branch attached to more than one worktree

[masawada]

Why

• git worktree add --force can attach a branch that is already checked out to a second worktree. eda remove then deletes the first worktree it finds and fails on git branch -D, leaving a partial state.
• The WorktreeRemove hook resolves only the branch name from worktree_path, so in that state it deletes a worktree other than the one it was given.

What

• Collect every linked worktree attached to the branch and refuse, before inspecting or removing an individual worktree, when there is more than one. The refusal is a refusalError, so the hook reports the worktrees as kept.
• Tests for the removal logic (duplicates inside and outside the worktree root) and the hook.