feat(mcp): 云端 MCP 工具从 8 个扩到 12 个

#159 只包了读 + 建 + 取消，日常闭环还缺：读产物文件、
读完整结果、重试、合并。新增 get_task_files（path 为
worktree 内相对路径，服务端 _safe_worktree_path 防逃逸，
MCP 层不展开绝对路径）、get_task_result_full（依赖 #162
的 /result-full 端点，未部署前该工具回 404 isError，不炸）、
retry_task、merge_task（写操作，服务端状态机校验，非法
转换 409 原样透传，MCP 层不绕过）。
单测 test_mcp_server 10 项（含新增翻译 1 项）。

cloud/tests/test_mcp_server.py

@@ -39,6 +39,16 @@ class FakeTransport:
             return 200, {"total_tokens": 10}
         if path == "/tasks/t1/trace":
             return 200, {"entries": []}
+        if path == "/tasks/t1/files":
+            return 200, {"files": [{"path": "a.py"}]}
+        if path == "/tasks/t1/files?path=a.py":
+            return 200, {"content": "print(1)"}
+        if path == "/tasks/t1/result-full":
+            return 200, {"result_text_full": "full-result"}
+        if path == "/tasks/t1/retry":
+            return 200, {"id": "t1", "status": "queued"}
+        if path == "/tasks/t1/merge":
+            return 200, {"id": "t1", "status": "merged"}
         if path == "/tasks/nope":
             return 404, {"error": "not found"}
         return 400, {"error": "bad request"}
@@ -111,6 +121,28 @@ class MCPServerTestCase(unittest.TestCase):
         payload = json.loads(resp["result"]["content"][0]["text"])
         self.assertEqual(payload["http_status"], 404)
 
+    def test_new_tools_translate_to_rest(self):
+        is_error, payload = self.server.call_tool(
+            "get_task_files", {"task_id": "t1"})
+        self.assertFalse(is_error)
+        self.assertEqual(payload["files"][0]["path"], "a.py")
+        is_error, payload = self.server.call_tool(
+            "get_task_files", {"task_id": "t1", "path": "a.py"})
+        self.assertFalse(is_error)
+        self.assertEqual(payload["content"], "print(1)")
+        is_error, payload = self.server.call_tool(
+            "get_task_result_full", {"task_id": "t1"})
+        self.assertFalse(is_error)
+        self.assertEqual(payload["result_text_full"], "full-result")
+        is_error, payload = self.server.call_tool(
+            "retry_task", {"task_id": "t1"})
+        self.assertFalse(is_error)
+        self.assertEqual(payload["status"], "queued")
+        is_error, payload = self.server.call_tool(
+            "merge_task", {"task_id": "t1"})
+        self.assertFalse(is_error)
+        self.assertEqual(payload["status"], "merged")
+
     def test_missing_arg_is_error_not_rest_call(self):
         before = len(self.transport.calls)
         is_error, payload = self.server.call_tool("list_tasks", {})