feat(workflow): add git.create_tag config to disable milestone tagging

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>

commands/gsd/complete-milestone.md

@@ -126,7 +126,7 @@ Output: Milestone archived (roadmap + requirements), PROJECT.md evolved, git tag
 - `.planning/REQUIREMENTS.md` deleted (fresh for next milestone)
 - ROADMAP.md collapsed to one-line entry
 - PROJECT.md updated with current state
-- Git tag v{{version}} created
+- Git tag v{{version}} created (if `git.create_tag` enabled)
 - Commit successful
 - User knows next steps (including need for fresh requirements)
   </success_criteria>