feat: agent definition v2 (registry, persistence, developerInstructions channel)

Registry: POST /tasks accepts agent_name, loading <name>.json
from repo-root agents/ (traversal-guarded, skill-pack re-validated);
agent and agent_name together is a 400. Persistence: tasks gains
agent_name/agent_version/agent_instructions (migrate + PG schema),
returned by GET /tasks; stored prompt stays clean. Channel: local
and docker dispatch pass --developer-instructions to host.py
dispatch -> create_thread(developerInstructions=...); run/fork do
not re-send. Cloud executor has no such channel, so the worker
falls back to the v1 marked header in the in-memory dispatch
prompt only (DB prompt stays clean). Goal/coordinator tasks
still reject instructions.

Tests: test_agent_def_v2.py (7 tests) + updated v1 suite.

cloud/tests/test_agent_def.py

@@ -1,4 +1,6 @@
-"""声明式 agent 定义 v1 单测：解析、校验、解析到任务字段、建任务入口。
+"""声明式 agent 定义 v1/v2 单测：解析、校验、解析到任务字段、建任务入口。
+
+v2 改动：prompt 不再拼头，指令落库（见 test_agent_def_v2.py）。
 
 运行（在仓库根目录下）：
     python3 -m unittest discover -s cloud/tests -p "test_agent_def.py" -v
@@ -168,7 +170,9 @@ class ServerAgentTestCase(unittest.TestCase):
         self.assertEqual(status, 200)
         return data["tasks"]
 
-    def test_agent_fills_fields_and_header(self):
+    def test_agent_fills_fields_and_stores_instructions(self):
+        # v2：prompt 保持干净，指令落库，不再拼头（v1 行为已
+        # 被 v2 替换，见 agent_def.py 的 v2 边界说明）。
         status, task = self._req("POST", "/tasks", {
             "project_id": self.project["id"], "prompt": "do research",
             "agent": _agent()}, self.owner_key)
@@ -179,10 +183,14 @@ class ServerAgentTestCase(unittest.TestCase):
         self.assertEqual(task["executor"], "docker")
         self.assertEqual(task["agent"]["name"], "research-brief")
         self.assertEqual(task["agent"]["skills"], ["live-info"])
+        self.assertEqual(task["agent_name"], "research-brief")
+        self.assertEqual(task["agent_version"], "v1")
+        self.assertEqual(task["agent_instructions"],
+                         "Cite sources and dates.")
         stored = self._list_tasks()[0]
-        self.assertTrue(stored["prompt"].startswith(
-            "[Agent: research-brief v1]\nCite sources and dates.\n\n"))
-        self.assertTrue(stored["prompt"].endswith("do research"))
+        self.assertEqual(stored["prompt"], "do research")
+        self.assertEqual(stored["agent_instructions"],
+                         "Cite sources and dates.")
 
     def test_explicit_body_wins_over_agent(self):
         status, task = self._req("POST", "/tasks", {