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

@@ -0,0 +1,66 @@
+---
+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