Opening a new git worktree of a repository that has a dev container
configuration re-prompted you to reopen in a dev container, even after
you'd clicked "Don't Show Again" in another worktree of the same repo.
The dismissal was keyed on each worktree's own absolute path, so sibling
worktrees never saw the stored choice.

This keys the dismissal on the repository's common Git directory
instead, which is shared across all linked git worktrees of the same
repo, so dismissing in one worktree now suppresses the prompt in the
others. Projects that aren't Git repositories fall back to the worktree
path as before.

Closes AI-357

Release Notes:

- Fixed the "Reopen in Dev Container" suggestion re-appearing in new git
worktrees after choosing "Don't Show Again"