feat(scripts): add --allow-existing-branch flag to create-new-feature (#1999)

* feat(scripts): add --allow-existing-branch flag to create-new-feature

Add an --allow-existing-branch / -AllowExistingBranch flag to both
bash and PowerShell create-new-feature scripts. When the target branch
already exists, the script switches to it instead of failing. The spec
directory and template are still created if missing, but existing
spec.md files are not overwritten (prevents data loss on re-runs).

The flag is opt-in, so existing behavior is completely unchanged
without it. This enables worktree-based workflows and CI/CD pipelines
that create branches externally before running speckit.specify.

Relates to #1931. Also addresses #1680, #841, #1921.

Assisted-By: 🤖 Claude Code

* fix: address PR review feedback for allow-existing-branch

- Make checkout failure fatal instead of suppressing with || true (bash)
- Check $LASTEXITCODE after git checkout in PowerShell
- Use Test-Path -PathType Leaf for spec file existence check (PS)
- Add PowerShell static assertion test for -AllowExistingBranch flag

Assisted-By: 🤖 Claude Code

tests/test_timestamp_branches.py

@@ -269,3 +269,146 @@ class TestE2EFlow:
         assert (git_repo / "specs" / branch).is_dir()
         val = source_and_call(f'check_feature_branch "{branch}" "true"')
         assert val.returncode == 0
+
+
+# ── Allow Existing Branch Tests ──────────────────────────────────────────────
+
+
+class TestAllowExistingBranch:
+    def test_allow_existing_switches_to_branch(self, git_repo: Path):
+        """T006: Pre-create branch, verify script switches to it."""
+        subprocess.run(
+            ["git", "checkout", "-b", "004-pre-exist"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        subprocess.run(
+            ["git", "checkout", "-"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        result = run_script(
+            git_repo, "--allow-existing-branch", "--short-name", "pre-exist",
+            "--number", "4", "Pre-existing feature",
+        )
+        assert result.returncode == 0, result.stderr
+        current = subprocess.run(
+            ["git", "rev-parse", "--abbrev-ref", "HEAD"],
+            cwd=git_repo, capture_output=True, text=True,
+        ).stdout.strip()
+        assert current == "004-pre-exist", f"expected 004-pre-exist, got {current}"
+
+    def test_allow_existing_already_on_branch(self, git_repo: Path):
+        """T007: Verify success when already on the target branch."""
+        subprocess.run(
+            ["git", "checkout", "-b", "005-already-on"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        result = run_script(
+            git_repo, "--allow-existing-branch", "--short-name", "already-on",
+            "--number", "5", "Already on branch",
+        )
+        assert result.returncode == 0, result.stderr
+
+    def test_allow_existing_creates_spec_dir(self, git_repo: Path):
+        """T008: Verify spec directory created on existing branch."""
+        subprocess.run(
+            ["git", "checkout", "-b", "006-spec-dir"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        subprocess.run(
+            ["git", "checkout", "-"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        result = run_script(
+            git_repo, "--allow-existing-branch", "--short-name", "spec-dir",
+            "--number", "6", "Spec dir feature",
+        )
+        assert result.returncode == 0, result.stderr
+        assert (git_repo / "specs" / "006-spec-dir").is_dir()
+        assert (git_repo / "specs" / "006-spec-dir" / "spec.md").exists()
+
+    def test_without_flag_still_errors(self, git_repo: Path):
+        """T009: Verify backwards compatibility (error without flag)."""
+        subprocess.run(
+            ["git", "checkout", "-b", "007-no-flag"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        subprocess.run(
+            ["git", "checkout", "-"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        result = run_script(
+            git_repo, "--short-name", "no-flag", "--number", "7", "No flag feature",
+        )
+        assert result.returncode != 0, "should fail without --allow-existing-branch"
+        assert "already exists" in result.stderr
+
+    def test_allow_existing_no_overwrite_spec(self, git_repo: Path):
+        """T010: Pre-create spec.md with content, verify it is preserved."""
+        subprocess.run(
+            ["git", "checkout", "-b", "008-no-overwrite"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        spec_dir = git_repo / "specs" / "008-no-overwrite"
+        spec_dir.mkdir(parents=True)
+        spec_file = spec_dir / "spec.md"
+        spec_file.write_text("# My custom spec content\n")
+        subprocess.run(
+            ["git", "checkout", "-"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        result = run_script(
+            git_repo, "--allow-existing-branch", "--short-name", "no-overwrite",
+            "--number", "8", "No overwrite feature",
+        )
+        assert result.returncode == 0, result.stderr
+        assert spec_file.read_text() == "# My custom spec content\n"
+
+    def test_allow_existing_creates_branch_if_not_exists(self, git_repo: Path):
+        """T011: Verify normal creation when branch doesn't exist."""
+        result = run_script(
+            git_repo, "--allow-existing-branch", "--short-name", "new-branch",
+            "New branch feature",
+        )
+        assert result.returncode == 0, result.stderr
+        current = subprocess.run(
+            ["git", "rev-parse", "--abbrev-ref", "HEAD"],
+            cwd=git_repo, capture_output=True, text=True,
+        ).stdout.strip()
+        assert "new-branch" in current
+
+    def test_allow_existing_with_json(self, git_repo: Path):
+        """T012: Verify JSON output is correct."""
+        import json
+
+        subprocess.run(
+            ["git", "checkout", "-b", "009-json-test"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        subprocess.run(
+            ["git", "checkout", "-"],
+            cwd=git_repo, check=True, capture_output=True,
+        )
+        result = run_script(
+            git_repo, "--allow-existing-branch", "--json", "--short-name", "json-test",
+            "--number", "9", "JSON test",
+        )
+        assert result.returncode == 0, result.stderr
+        data = json.loads(result.stdout)
+        assert data["BRANCH_NAME"] == "009-json-test"
+
+    def test_allow_existing_no_git(self, no_git_dir: Path):
+        """T013: Verify flag is silently ignored in non-git repos."""
+        result = run_script(
+            no_git_dir, "--allow-existing-branch", "--short-name", "no-git",
+            "No git feature",
+        )
+        assert result.returncode == 0, result.stderr
+
+
+class TestAllowExistingBranchPowerShell:
+    def test_powershell_supports_allow_existing_branch_flag(self):
+        """Static guard: PS script exposes and uses -AllowExistingBranch."""
+        contents = CREATE_FEATURE_PS.read_text(encoding="utf-8")
+        assert "-AllowExistingBranch" in contents
+        # Ensure the flag is referenced in script logic, not just declared
+        assert "AllowExistingBranch" in contents.replace("-AllowExistingBranch", "")