Follow-up to #58996. The "Remove Worktree from Window" button captured
the row index at render time and re-resolved it against the picker's
matches at click time. Matches are rebuilt asynchronously (initial
worktree list load, post-delete refresh, query changes), so the index
could drift between the rendered frame and the click being processed,
closing the workspace of a different worktree than the one the user
clicked.

This changes the click handler to capture the worktree's path — a stable
identity — instead of its index, adds the same `deleting_worktree_paths`
click-time guard the neighboring handlers use, and constructs the button
only when it will actually be shown.

Scoped to the code added in #58996; the same stale-index pattern in the
delete and open-in-new-window handlers is left for a later change.

Release Notes:

- Fixed an issue where the worktree picker's "Remove Worktree from
Window" button could act on the wrong worktree if the list updated at
the moment of the click.