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/commands/speckit.git.feature.md

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+---
+description: "Create a feature branch with sequential or timestamp numbering"
+---
+
+# Create Feature Branch
+
+Create a new feature branch for the given specification.
+
+## User Input
+
+```text
+$ARGUMENTS
+```
+
+You **MUST** consider the user input before proceeding (if not empty).
+
+## Prerequisites
+
+- Verify Git is available by running `git rev-parse --is-inside-work-tree 2>/dev/null`
+- If Git is not available, warn the user and skip branch creation (spec directory will still be created)
+
+## Branch Numbering Mode
+
+Determine the branch numbering strategy by checking configuration in this order:
+
+1. Check `.specify/extensions/git/git-config.yml` for `branch_numbering` value
+2. Check `.specify/init-options.json` for `branch_numbering` value (backward compatibility)
+3. Default to `sequential` if neither exists
+
+## Execution
+
+Generate a concise short name (2-4 words) for the branch:
+- Analyze the feature description and extract the most meaningful keywords
+- Use action-noun format when possible (e.g., "add-user-auth", "fix-payment-bug")
+- Preserve technical terms and acronyms (OAuth2, API, JWT, etc.)
+
+Run the appropriate script based on your platform:
+
+- **Bash**: `.specify/extensions/git/scripts/bash/create-new-feature.sh --json --short-name "<short-name>" "<feature description>"`
+- **Bash (timestamp)**: `.specify/extensions/git/scripts/bash/create-new-feature.sh --json --timestamp --short-name "<short-name>" "<feature description>"`
+- **PowerShell**: `.specify/extensions/git/scripts/powershell/create-new-feature.ps1 -Json -ShortName "<short-name>" "<feature description>"`
+- **PowerShell (timestamp)**: `.specify/extensions/git/scripts/powershell/create-new-feature.ps1 -Json -Timestamp -ShortName "<short-name>" "<feature description>"`
+
+**IMPORTANT**:
+- Do NOT pass `--number` — the script determines the correct next number automatically
+- Always include the JSON flag (`--json` for Bash, `-Json` for PowerShell) so the output can be parsed reliably
+- You must only ever run this script once per feature
+- The JSON output will contain BRANCH_NAME and SPEC_FILE paths
+
+If the extension scripts are not found at the `.specify/extensions/git/` path, fall back to:
+- **Bash**: `scripts/bash/create-new-feature.sh`
+- **PowerShell**: `scripts/powershell/create-new-feature.ps1`
+
+## Graceful Degradation
+
+If Git is not installed or the current directory is not a Git repository:
+- The script will still create the spec directory under `specs/`
+- A warning will be printed: `[specify] Warning: Git repository not detected; skipped branch creation`
+- The workflow continues normally without branch creation
+
+## Output
+
+The script outputs JSON with:
+- `BRANCH_NAME`: The created branch name (e.g., `003-user-auth` or `20260319-143022-user-auth`)
+- `SPEC_FILE`: Path to the created spec file
+- `FEATURE_NUM`: The numeric or timestamp prefix used

extensions/git/commands/speckit.git.remote.md

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+---
+description: "Detect Git remote URL for GitHub integration"
+---
+
+# Detect Git Remote URL
+
+Detect the Git remote URL for integration with GitHub services (e.g., issue creation).
+
+## Prerequisites
+
+- Check if Git is available by running `git rev-parse --is-inside-work-tree 2>/dev/null`
+- If Git is not available, output a warning and return empty:
+  ```
+  [specify] Warning: Git repository not detected; cannot determine remote URL
+  ```
+
+## Execution
+
+Run the following command to get the remote URL:
+
+```bash
+git config --get remote.origin.url
+```
+
+## Output
+
+Parse the remote URL and determine:
+
+1. **Repository owner**: Extract from the URL (e.g., `github` from `https://github.com/github/spec-kit.git`)
+2. **Repository name**: Extract from the URL (e.g., `spec-kit` from `https://github.com/github/spec-kit.git`)
+3. **Is GitHub**: Whether the remote points to a GitHub repository
+
+Supported URL formats:
+- HTTPS: `https://github.com/<owner>/<repo>.git`
+- SSH: `git@github.com:<owner>/<repo>.git`
+
+> [!CAUTION]
+> ONLY report a GitHub repository if the remote URL actually points to github.com.
+> Do NOT assume the remote is GitHub if the URL format doesn't match.
+
+## Graceful Degradation
+
+If Git is not installed, the directory is not a Git repository, or no remote is configured:
+- Return an empty result
+- Do NOT error — other workflows should continue without Git remote information

extensions/git/commands/speckit.git.validate.md

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+---
+description: "Validate current branch follows feature branch naming conventions"
+---
+
+# Validate Feature Branch
+
+Validate that the current Git branch follows the expected feature branch naming conventions.
+
+## Prerequisites
+
+- Check if Git is available by running `git rev-parse --is-inside-work-tree 2>/dev/null`
+- If Git is not available, output a warning and skip validation:
+  ```
+  [specify] Warning: Git repository not detected; skipped branch validation
+  ```
+
+## Validation Rules
+
+Get the current branch name:
+
+```bash
+git rev-parse --abbrev-ref HEAD
+```
+
+The branch name must match one of these patterns:
+
+1. **Sequential**: `^[0-9]{3}-` (e.g., `001-feature-name`, `042-fix-bug`)
+2. **Timestamp**: `^[0-9]{8}-[0-9]{6}-` (e.g., `20260319-143022-feature-name`)
+
+## Execution
+
+If on a feature branch (matches either pattern):
+- Output: `✓ On feature branch: <branch-name>`
+- Check if the corresponding spec directory exists under `specs/`:
+  - For sequential branches, look for `specs/<prefix>-*` where prefix matches the `###` portion
+  - For timestamp branches, look for `specs/<prefix>-*` where prefix matches the `YYYYMMDD-HHMMSS` portion
+- If spec directory exists: `✓ Spec directory found: <path>`
+- If spec directory missing: `⚠ No spec directory found for prefix <prefix>`
+
+If NOT on a feature branch:
+- Output: `✗ Not on a feature branch. Current branch: <branch-name>`
+- Output: `Feature branches should be named like: 001-feature-name or 20260319-143022-feature-name`
+
+## Graceful Degradation
+
+If Git is not installed or the directory is not a Git repository:
+- Check the `SPECIFY_FEATURE` environment variable as a fallback
+- If set, validate that value against the naming patterns
+- If not set, skip validation with a warning