When running bd init --stealth in a worktree, excludes were being
written to .git/worktrees/<name>/info/exclude which has no effect.
Changed setupGitExclude and setupForkExclude to use --git-common-dir
instead of --git-dir so excludes go to the main repo .git/info/exclude.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>