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>

docs/CONFIGURATION.md

@@ -542,6 +542,7 @@ All four fields are **optional and additive** — STATE.md files without them ke
 |---------|------|---------|-------------|
 | `git.branching_strategy` | enum | `none` | `none`, `phase`, or `milestone` |
 | `git.base_branch` | string | `main` | The integration branch that phase/milestone branches are created from and merged back into. Override when your repo uses `master` or a release branch |
+| `git.create_tag` | boolean | `true` | Create a git tag (`v[X.Y]`) on milestone completion. Set to `false` for projects with their own release flow |
 | `git.phase_branch_template` | string | `gsd/phase-{phase}-{slug}` | Branch name template for phase strategy |
 | `git.milestone_branch_template` | string | `gsd/{milestone}-{slug}` | Branch name template for milestone strategy |
 | `git.quick_branch_template` | string or null | `null` | Optional branch name template for `/gsd-quick` tasks |