feat: 新建任务的 prompt 里写 /goal 也自动变 goal

上一提交只覆盖了已有主线的转换，新建任务的 prompt 里写
/goal 时仍会当成普通一轮任务跑。本提交补上 POST /tasks
入口：prompt 经 cloud/goal_command.py 解析后，目标即新任
务的 prompt 与 goal，goal_mode 强制为 1（命令优先于 body
里的 goal_mode=0），plan/验收命令冲突仍按新建规则 400，
空目标 400 不建任务；讨论里提到 /goal 不触发。
单测补 5 项：/goal 直写、整句、空目标、plan/验收冲突、
讨论 prompt 不变。

cloud/tests/test_mainline_goal_command.py

@@ -208,6 +208,46 @@ class ServerTestCase(unittest.TestCase):
         after = self._get_task(task["id"])
         self.assertEqual(after["goal_mode"], 0)
 
+    def test_create_task_prompt_goal_command(self):
+        task = self._create(prompt="/goal 收xxx")
+        self.assertEqual(task["goal_mode"], 1)
+        self.assertEqual(task["goal"], "收xxx")
+        self.assertEqual(task["prompt"], "收xxx")
+        self.assertIsNone(task["verify_command"])
+        self.assertEqual(task["goal_max_rounds"], 0)
+
+    def test_create_task_prompt_goal_user_sentence(self):
+        task = self._create(
+            prompt="请在这条主线开启一个持久 /goal，目标是收xxx")
+        self.assertEqual(task["goal_mode"], 1)
+        self.assertEqual(task["goal"], "收xxx")
+        self.assertEqual(task["prompt"], "收xxx")
+
+    def test_create_task_prompt_goal_empty_400(self):
+        status, body = self._req(
+            "POST", "/tasks",
+            {"project_id": self.project["id"], "prompt": "/goal"},
+            self.owner_key)
+        self.assertEqual(status, 400, body)
+
+    def test_create_task_prompt_goal_conflicts_400(self):
+        status, _body = self._req(
+            "POST", "/tasks",
+            {"project_id": self.project["id"], "prompt": "/goal 收xxx",
+             "plan": 1},
+            self.owner_key)
+        self.assertEqual(status, 400)
+        status, _body = self._req(
+            "POST", "/tasks",
+            {"project_id": self.project["id"], "prompt": "/goal 收xxx",
+             "verify_command": "true"},
+            self.owner_key)
+        self.assertEqual(status, 400)
+
+    def test_create_task_discussion_prompt_unchanged(self):
+        task = self._create(prompt="讨论/goal的用法")
+        self.assertEqual(task["goal_mode"], 0)
+
     def test_continue_goal_command(self):
         task = self._create()
         self._set_status(task["id"], "completed")