Adds boolean config key `git.create_tag` (default: true, fully backcompat) so projects with their own release flow can disable GSD's automatic `git tag -a v[X.Y]` on milestone completion. Also adds tag-collision pre-check to prevent silent failure on re-run. Closes #3086 Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>