feat(scripts): optional single-segment branch prefix for gitflow (#2202)

* feat(scripts): optional single-segment branch prefix for gitflow

- Add spec_kit_effective_branch_name / Get-SpecKitEffectiveBranchName:
  when branch matches prefix/rest with exactly one slash, validate and
  resolve specs/ using only the rest (e.g. feat/001-my-feature).
- Wire into check_feature_branch, find_feature_dir_by_prefix (bash) and
  Test-FeatureBranch, Find-FeatureDirByPrefix + Get-FeaturePathsEnv (PS).
- Align git extension git-common with core validation; remove unused
  get_feature_dir / Get-FeatureDir helpers.
- Extend tests in test_timestamp_branches.py and test_git_extension.py.

Made-with: Cursor

* Update scripts/powershell/common.ps1

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix(ps): align feature-dir resolution errors with bash (no throw under Stop)

Find-FeatureDirByPrefix: on ambiguous prefix matches, write errors to stderr
and return $null instead of throwing, matching find_feature_dir_by_prefix.

Get-FeaturePathsEnv: narrow try/catch to ConvertFrom-Json only; add
Get-FeatureDirFromBranchPrefixOrExit to mirror bash get_feature_paths
(stderr + exit 1) when prefix lookup fails, avoiding unhandled terminating
errors under $ErrorActionPreference = 'Stop' in check-prerequisites,
setup-plan, and update-agent-context.

Made-with: Cursor

* Update tests/test_timestamp_branches.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update extensions/git/scripts/powershell/git-common.ps1

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update scripts/powershell/common.ps1

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

tests/extensions/git/test_git_extension.py

@@ -587,3 +587,40 @@ class TestGitCommonBash:
             capture_output=True, text=True,
         )
         assert result.returncode != 0
+
+    def test_check_feature_branch_accepts_single_prefix(self, tmp_path: Path):
+        """git-common check_feature_branch matches core: one optional path prefix."""
+        project = _setup_project(tmp_path)
+        script = project / ".specify" / "extensions" / "git" / "scripts" / "bash" / "git-common.sh"
+        result = subprocess.run(
+            ["bash", "-c", f'source "{script}" && check_feature_branch "feat/001-my-feature" "true"'],
+            capture_output=True, text=True,
+        )
+        assert result.returncode == 0
+
+    def test_check_feature_branch_rejects_nested_prefix(self, tmp_path: Path):
+        project = _setup_project(tmp_path)
+        script = project / ".specify" / "extensions" / "git" / "scripts" / "bash" / "git-common.sh"
+        result = subprocess.run(
+            ["bash", "-c", f'source "{script}" && check_feature_branch "feat/fix/001-x" "true"'],
+            capture_output=True, text=True,
+        )
+        assert result.returncode != 0
+
+
+@pytest.mark.skipif(not HAS_PWSH, reason="pwsh not available")
+class TestGitCommonPowerShell:
+    def test_test_feature_branch_accepts_single_prefix(self, tmp_path: Path):
+        project = _setup_project(tmp_path)
+        script = project / ".specify" / "extensions" / "git" / "scripts" / "powershell" / "git-common.ps1"
+        result = subprocess.run(
+            [
+                "pwsh",
+                "-NoProfile",
+                "-Command",
+                f'. "{script}"; if (Test-FeatureBranch -Branch "feat/001-x" -HasGit $true) {{ exit 0 }} else {{ exit 1 }}',
+            ],
+            capture_output=True,
+            text=True,
+        )
+        assert result.returncode == 0