feat: create extensions/git with manifest, commands, scripts, and auto-install in init

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>
Agent-Logs-Url: https://github.com/github/spec-kit/sessions/809a1dbf-1301-4312-b4d2-e18f9b3e8b2f

extensions/git/README.md

@@ -0,0 +1,85 @@
+# Git Branching Workflow Extension
+
+Feature branch creation, numbering (sequential/timestamp), validation, and Git remote detection for Spec Kit.
+
+## Overview
+
+This extension provides Git branching operations as an optional, self-contained module. It manages:
+
+- **Feature branch creation** with sequential (`001-feature-name`) or timestamp (`20260319-143022-feature-name`) numbering
+- **Branch validation** to ensure branches follow naming conventions
+- **Git remote detection** for GitHub integration (e.g., issue creation)
+
+## Commands
+
+| Command | Description |
+|---------|-------------|
+| `speckit.git.feature` | Create a feature branch with sequential or timestamp numbering |
+| `speckit.git.validate` | Validate current branch follows feature branch naming conventions |
+| `speckit.git.remote` | Detect Git remote URL for GitHub integration |
+
+## Hooks
+
+| Event | Command | Optional | Description |
+|-------|---------|----------|-------------|
+| `before_specify` | `speckit.git.feature` | No | Create feature branch before specification |
+| `after_implement` | `speckit.git.validate` | Yes | Validate branch naming after implementation |
+
+## Configuration
+
+Configuration is stored in `.specify/extensions/git/git-config.yml`:
+
+```yaml
+# Branch numbering strategy: "sequential" or "timestamp"
+branch_numbering: sequential
+
+# Branch name template
+branch_template: "{number}-{short_name}"
+
+# Whether to fetch remotes before computing next branch number
+auto_fetch: true
+```
+
+### Environment Variable Override
+
+Set `SPECKIT_GIT_BRANCH_NUMBERING` to override the `branch_numbering` config value:
+
+```bash
+export SPECKIT_GIT_BRANCH_NUMBERING=timestamp
+```
+
+## Installation
+
+```bash
+# Install from the bundled extension
+specify extension add git --from extensions/git/
+
+# Or it auto-installs during specify init (migration period)
+```
+
+## Disabling
+
+```bash
+# Disable the git extension (spec creation continues without branching)
+specify extension disable git
+
+# Re-enable it
+specify extension enable git
+```
+
+## Graceful Degradation
+
+When Git is not installed or the directory is not a Git repository:
+- Spec directories are still created under `specs/`
+- Branch creation is skipped with a warning
+- Branch validation is skipped with a warning
+- Remote detection returns empty results
+
+## Scripts
+
+The extension bundles cross-platform scripts:
+
+- `scripts/bash/create-new-feature.sh` — Bash implementation
+- `scripts/bash/git-common.sh` — Shared Git utilities (Bash)
+- `scripts/powershell/create-new-feature.ps1` — PowerShell implementation
+- `scripts/powershell/git-common.ps1` — Shared Git utilities (PowerShell)