test(remove): pin the ownership gate's refusal of an emptied worktree directory

[max-sixty]

Removal's ownership gate (ensure_holds_this_worktree, #3808) reads <dir>/.git and stops there, so a directory that has lost that entry holds no worktree of ours and is refused. Nothing named that: wt remove reaches the same refusal upstream at the prunable check, and the gate's own tests cover the sibling-worktree and foreign-repo cases instead.

The case bites where the emptied directory sits inside a repository — a worktree nested in another, or any worktree under a git-managed ~. git rev-parse --git-dir walks up from it and answers with the enclosing repository's git dir, which is the common dir, so a gate resolving that way reads the directory as the main worktree and accepts it:

$ git -C repo worktree add nested -b nested && rm repo/nested/.git
$ git -C repo/nested rev-parse --git-dir
/…/repo/.git

So the test nests, and asserts that premise before asserting the refusal — otherwise it would go vacuous if the nesting stopped producing that resolution. In the flat sibling layout the walk-up finds no repository at all and resolution simply fails, which is why a sibling-shaped test would pin nothing.

Confirmed discriminating: with resolution swapped back to the GIT_DIRS-cached git_dir(), it fails while ensure_holds_this_worktree_accepts_both_worktree_shapes still passes.

This was written by Claude Code on behalf of max-sixty