GRM-35: fix: strip git-cliff header from CHANGELOG.md updates

The update_changelog function now strips the git-cliff header before inserting into CHANGELOG.md, preventing duplicate headers.

Closes GRM-35

tests/unit/test_cli.py

@@ -12,7 +12,7 @@ class TestCLI:
         runner = CliRunner()
         result = runner.invoke(cli, ["--version"])
         assert result.exit_code == 0
-        assert "0.1.0" in result.output
+        assert "0.2.0" in result.output
 
     @patch("gitea_runner_manager.cli.RunnerManager")
     def test_install(self, mock_manager_class: MagicMock) -> None:

tests/unit/test_release.py

@@ -186,6 +186,30 @@ class TestUpdateChangelog:
         assert "Some intro text" in content
         assert "## [0.2.0]" in content
 
+    def test_strips_git_cliff_header(self, tmp_path, monkeypatch) -> None:
+        """git-cliff output includes a header — should be stripped before inserting."""
+        changelog_file = tmp_path / "CHANGELOG.md"
+        changelog_file.write_text("# Changelog\n\n## [0.1.0] - 2026-06-20\n\n### Features\n- old thing\n")
+        monkeypatch.setattr("scripts.release.CHANGELOG_FILE", str(changelog_file))
+        # Simulate git-cliff output with header
+        cliff_output = "# Changelog\n\nAll notable changes...\n\n## [0.2.0] - 2026-06-21\n\n### Features\n- new thing"
+        update_changelog(cliff_output)
+        content = changelog_file.read_text()
+        # Header should appear only once (from the existing file)
+        assert content.count("# Changelog") == 1
+        assert "## [0.2.0]" in content
+        assert "new thing" in content
+
+    def test_strips_header_when_creating_new_file(self, tmp_path, monkeypatch) -> None:
+        """When creating a new file, strip the git-cliff header."""
+        changelog_file = tmp_path / "CHANGELOG.md"
+        monkeypatch.setattr("scripts.release.CHANGELOG_FILE", str(changelog_file))
+        cliff_output = "# Changelog\n\nAll notable changes...\n\n## [0.2.0] - 2026-06-21\n\n### Features\n- new thing"
+        update_changelog(cliff_output)
+        content = changelog_file.read_text()
+        assert "# Changelog" not in content
+        assert "## [0.2.0]" in content
+
 
 class TestCommitReleaseChanges:
     @patch("scripts.release.run_cmd")