fix: address all PR review comments for git extension

- Fix bash common.sh sourcing: check .specify/scripts/bash/ first, then
  scripts/bash/, with explicit error if neither found
- Fix PowerShell common.ps1 sourcing: use $fallbackRoot for reliable
  path resolution, with explicit error if not found
- Remove undocumented branch_template and auto_fetch from extension.yml
  defaults and config-template.yml (scripts don't use them yet)
- Remove unused ExtensionError import in _install_bundled_git_extension
- Remove undocumented SPECKIT_GIT_BRANCH_NUMBERING env var from README
- Fix specify.md: skip step 2 when before_specify hook already executed
- Fix specify.md: add explicit FEATURE_DIR/SPEC_FILE in disabled-git path
- Fix specify.md: add PowerShell path to script resolution
- Add tests for git extension auto-install during specify init

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>
Agent-Logs-Url: https://github.com/github/spec-kit/sessions/008835a0-7778-40bb-bdb2-4182b22be315

tests/test_branch_numbering.py

@@ -87,3 +87,54 @@ class TestBranchNumberingValidation:
         result = runner.invoke(app, ["init", str(tmp_path / "proj"), "--ai", "claude", "--branch-numbering", "timestamp", "--ignore-agent-tools"])
         assert result.exit_code == 0
         assert "Invalid --branch-numbering" not in (result.output or "")
+
+
+class TestGitExtensionAutoInstall:
+    """Tests for bundled git extension auto-install during specify init."""
+
+    def test_git_extension_installed_during_init(self, tmp_path: Path, monkeypatch):
+        """verify that `specify init` auto-installs the bundled git extension."""
+        from typer.testing import CliRunner
+        from specify_cli import app
+
+        def _fake_download(project_path, *args, **kwargs):
+            Path(project_path).mkdir(parents=True, exist_ok=True)
+
+        monkeypatch.setattr("specify_cli.download_and_extract_template", _fake_download)
+
+        project_dir = tmp_path / "proj"
+        runner = CliRunner()
+        result = runner.invoke(app, ["init", str(project_dir), "--ai", "claude", "--ignore-agent-tools"])
+        assert result.exit_code == 0
+
+        # Extension files should exist
+        ext_dir = project_dir / ".specify" / "extensions" / "git"
+        assert ext_dir.is_dir(), "git extension directory not created"
+        assert (ext_dir / "extension.yml").is_file(), "extension.yml not installed"
+
+        # Registry should contain the git extension
+        registry_file = project_dir / ".specify" / "extensions" / ".registry"
+        assert registry_file.is_file(), "extension registry not created"
+        registry = json.loads(registry_file.read_text())
+        assert "git" in registry.get("extensions", {}), "git not in registry"
+        assert registry["extensions"]["git"]["enabled"] is True
+
+    def test_git_extension_noop_when_already_installed(self, tmp_path: Path):
+        """_install_bundled_git_extension should no-op if git is already installed."""
+        from specify_cli import _install_bundled_git_extension
+        from specify_cli.extensions import ExtensionManager
+
+        project_dir = tmp_path / "proj"
+        (project_dir / ".specify").mkdir(parents=True)
+
+        # First install
+        result1 = _install_bundled_git_extension(project_dir)
+        assert result1 is True
+
+        # Second install should also succeed (no-op)
+        result2 = _install_bundled_git_extension(project_dir)
+        assert result2 is True
+
+        # Only one entry in registry
+        manager = ExtensionManager(project_dir)
+        assert manager.registry.is_installed("git")