GRM-58: fix: enforce commit message convention on master with CI validation

tests/unit/test_validate_commit_msg.py

@@ -128,6 +128,29 @@ class TestMain:
         result = runner.invoke(main, [])
         assert result.exit_code == 2
 
+    def test_branch_override_accepts_master_commit(self) -> None:
+        """--branch master overrides branch detection (for CI use)."""
+        msg_path = self._write_msg("GRM-19: feat: add feature")
+        runner = CliRunner()
+        result = runner.invoke(main, [msg_path, "--branch", "master"])
+        assert result.exit_code == 0
+
+    def test_branch_override_rejects_missing_task_id(self) -> None:
+        """--branch master still enforces GRM-N: prefix."""
+        msg_path = self._write_msg("feat: add feature")
+        runner = CliRunner()
+        result = runner.invoke(main, [msg_path, "--branch", "master"])
+        assert result.exit_code == 1
+        assert "task ID" in result.output
+
+    def test_branch_override_feature_accepts_conventional(self) -> None:
+        """--branch feature still rejects GRM-N prefix."""
+        msg_path = self._write_msg("GRM-19: feat: add feature")
+        runner = CliRunner()
+        result = runner.invoke(main, [msg_path, "--branch", "feature"])
+        assert result.exit_code == 1
+        assert "task ID" in result.output
+
 
 def test_main_module_block() -> None:
     import tempfile