feat: align mutation guard with runtime_context workspace resolution (Closes #460)

Share canonical workspace/repo-root resolution between gitea_get_runtime_context
and verify_preflight_purity so valid branches/ worktrees are not rejected when
the MCP process root differs from the stable control checkout. Fix relative
git-common-dir resolution and add regression tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

tests/test_create_issue_workspace_guard.py

@@ -106,20 +106,32 @@ class TestCreateIssueWorkspaceGuard(unittest.TestCase):
     @patch("gitea_mcp_server.issue_lock_worktree.read_worktree_git_state", return_value={"current_branch": "feat/issue-1"})
     @patch("os.path.exists", return_value=True)
     @patch("os.path.isdir", return_value=True)
+    @patch("author_mutation_worktree.subprocess.run")
     @patch("subprocess.run")
-    def test_create_issue_wrong_repo_fails_closed(self, mock_run, mock_isdir, mock_exists, _git, _get_all, mock_api, _role, _ns, _prof, _auth):
-        # Mock subprocess.run for git --git-common-dir to return a different path
-        mock_res = MagicMock()
-        mock_res.stdout = "/Users/jasonwalker/Development/some-other-repo/.git\n"
-        mock_run.return_value = mock_res
-
+    def test_create_issue_wrong_repo_fails_closed(self, mock_run, mock_amw_run, mock_isdir, mock_exists, _git, _get_all, mock_api, _role, _ns, _prof, _auth):
         wrong_repo_path = os.path.join(CONTROL_CHECKOUT_ROOT, "branches", "feat-issue-1")
 
+        def _subprocess_side_effect(cmd, *args, **kwargs):
+            mock_res = MagicMock(returncode=0)
+            if "--git-common-dir" in cmd:
+                cwd = cmd[cmd.index("-C") + 1] if "-C" in cmd else ""
+                if cwd == wrong_repo_path:
+                    mock_res.stdout = "/Users/jasonwalker/Development/some-other-repo/.git\n"
+                else:
+                    mock_res.stdout = f"{CONTROL_CHECKOUT_ROOT}/.git\n"
+            else:
+                mock_res.stdout = ""
+            return mock_res
+
+        mock_run.side_effect = _subprocess_side_effect
+        mock_amw_run.side_effect = _subprocess_side_effect
+
         with patch.object(srv, "PROJECT_ROOT", CONTROL_CHECKOUT_ROOT):
-            with self.assertRaises(RuntimeError) as ctx:
-                srv.gitea_create_issue(
-                    title="Test issue", body="body", worktree_path=wrong_repo_path
-                )
+            with patch("gitea_mcp_server._get_workspace_porcelain", return_value=""):
+                with self.assertRaises(RuntimeError) as ctx:
+                    srv.gitea_create_issue(
+                        title="Test issue", body="body", worktree_path=wrong_repo_path
+                    )
         self.assertIn("does not belong to the target repository", str(ctx.exception))
 
     @patch("gitea_mcp_server._auth", return_value=FAKE_AUTH)