feat(tasks): 完整结果另存 result_text_full，详情不再只剩 2000 字

旧链路三层截断：sandbox stdout 截 2000 字、worker 写回截
4000 字，长答案尾部在详情里永远查不到（只把上限调大是
把截断点后移，不是修掉）。修法：截断版保持不变（列表/
摘要用），完整版另存：
- sandbox _result_from_runner/_parse_host_result（local 与
  docker dispatch 路）回包新增 stdout_full（上限 200000 字，
  只防无界增长）；goal 路结论在尾部、tail 已保结论，不动。
- worker _write_execution_result 新增 result_text_full 列
  （_ensure_result_columns + server migrate_tasks_table 幂等
  补列），终局与取消分支均透传 stdout_full。
- server：任务 dict 加 result_full_available（不直接塞全文
  进列表负载），新增 GET /tasks/{id}/result-full（read 权限
  同款校验，无完整版时 404，不回摘要冒充完整）。
单测：test_queue_sandbox 45 项（含新增 stdout_full 1 项）、
test_marker_tools 30 项。

cloud/server.py

@@ -1104,6 +1104,7 @@ def migrate_tasks_table(conn):
         "PRAGMA table_info(tasks)").fetchall()}
     new_cols = {
         "result_text": "TEXT",
+        "result_text_full": "TEXT",
         "error_text": "TEXT",
         "actual_model": "TEXT",
         "started_at": "REAL",
@@ -1748,6 +1749,7 @@ def _task_to_dict(row, attachments=None, total_tokens=None):
         "updated_at": row["updated_at"],
         "last_heartbeat": row["last_heartbeat"],
         "result_text": _row_get(row, "result_text"),
+        "result_full_available": bool(_row_get(row, "result_text_full")),
         "error_text": _row_get(row, "error_text"),
         "actual_model": _row_get(row, "actual_model"),
         "started_at": _row_get(row, "started_at"),
@@ -2277,6 +2279,9 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
                     user_id, rest[: -len("/trace/stream")])
             elif rest.endswith("/trace") and method == "GET":
                 self._task_trace(user_id, rest[: -len("/trace")])
+            elif rest.endswith("/result-full") and method == "GET":
+                self._task_result_full(user_id,
+                                       rest[: -len("/result-full")])
             elif rest.endswith("/agents") and method == "GET":
                 self._task_agents(user_id, rest[: -len("/agents")])
             elif rest.endswith("/cancel") and method == "POST":
@@ -4626,6 +4631,28 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
                  for d in _trace_dirs_for_task(self.server, task)]
         return _read_trace_entries_multi(paths), ""
 
+    def _task_result_full(self, user_id, task_id):
+        """完整结果端点：result_text 仍是 4000 字摘要，完整版
+        在 result_text_full 里；无完整版时 404（老任务、goal
+        任务、或本轮没拿到 stdout_full 的任务），不回摘要
+        冒充完整。权限与 _get_task 同款 read 校验。"""
+        row = self._fetch_task(task_id)
+        if row is None:
+            self._send_json(404, {"error": "task not found"})
+            return
+        task = _task_to_dict(row)
+        with _auth_store(self.server.db_path) as store:
+            if not store.check_permission(user_id, task["project_id"], "read"):
+                self._send_json(403, {"error": "forbidden"})
+                return
+        full = _row_get(row, "result_text_full")
+        if not full:
+            self._send_json(404, {"error": "result full not available"})
+            return
+        self._send_json(200, {"task_id": task_id,
+                              "status": task["status"],
+                              "result_text_full": full})
+
     def _task_trace(self, user_id, task_id):
         row = self._fetch_task(task_id)
         if row is None: