Use worktrees for local core taps

- Avoid cloning `homebrew/core` and `homebrew/cask` when a brew
  worktree already has those repositories in a related checkout.
- Keep contributor worktree setups efficient by sharing local Git object
  storage instead of creating another network-backed checkout.
- Do not require `--force` when the local source tap is available,
  because the tap can be created without a remote clone.
- Preserve explicit cask clone targets so custom remotes keep their
  existing meaning.
- Remove Git worktree metadata on untap so repeated taps do not need
  manual worktree pruning.
- Resolve update marker paths through Git so linked worktrees do not
  print noisy `rm` errors for `.git` file checkouts.