feat: 任务间互发消息（API + agent 标记块，不中断任务）

同项目任务互发消息、对方做完可回消息：API POST /tasks/<目标>/messages
（from_task_id，目标项目 create_task + 发送项目 read 鉴权）与 agent
回合输出标记块（worker 回合后解析投递，不用联网、不用凭证，本地
与 Docker 都通）。投递不中断任务：对方运行中/排队中排成后续消息
（轮次边界执行，direct 优先）；对方终态则续跑复活（resume_message +
queued），A→B→A 往返不受回合时序影响。拒绝项：自己、跨项目、goal
目标、已合并、复活位被占；每轮最多 5 块，仅 completed 轮次投递；
目标执行时配额门照常生效。控制台后续消息列显示发送方，新增给
其他任务发消息框（落已代理的 /tasks 下，不用改 nginx）。

测试：新增 test_peer_messages（15）、test_peer_messages_server（8）、
test_worker_peer（4）；全量 cloud 654、host 64、deploy 19 全过（cloud 4 跳过）。

cloud/worker.py

@@ -137,6 +137,9 @@ from cloud.cost import CostStore, estimate_tokens, price_usd_micros, sum_usage
 from cloud.db import connect as _db_connect
 from cloud.logging_config import get_logger, log_event, setup_logging
 from cloud.models import ModelRouter, is_fallbackable, record_fallback
+from cloud.peer_messages import deliver_peer_message as _deliver_peer_message
+from cloud.peer_messages import peer_message_prefix as _peer_prefix
+from cloud.peer_messages import row_get as _row_get
 from cloud.pr import PrSkipped, PrStepError
 from cloud.pr_exec import (claim_pr_job, execute_task_pr, finish_pr_job)
 from cloud.queue import budget_exceeded, ready_tasks
@@ -1672,7 +1675,8 @@ class Worker:
             if cur.rowcount != 1:
                 return None
             return {"id": row["id"], "body": row["body"],
-                    "mode": row["mode"]}
+                    "mode": row["mode"],
+                    "sender_task_id": _row_get(row, "sender_task_id")}
         except Exception:
             return None
         finally:
@@ -1707,6 +1711,46 @@ class Worker:
         except Exception:
             pass
 
+    def _deliver_peer_messages(self, task, result):
+        """投递本轮 agent 输出里的互发消息标记块（见 peer_messages）。
+
+        只投递 completed 轮次的块：失败 / 超时 / 取消 / 配额中断
+        的轮次不投递；任务执行期间被取消时也不投递。投递本身
+        不中断任何任务：对方运行中/排队中排成后续消息（轮次边
+        界执行），对方终态则续跑复活；拒绝（自己、跨项目、
+        goal 目标、已合并、复活位被占等）只记审计，不炸 run_one。
+        投递不查发送方配额（发送本身不花钱），目标任务执行时
+        的配额门照常生效——互发消息不构成配额绕过。"""
+        if not result or result.get("status") != "completed":
+            return
+        if self._was_cancelled(task["id"]):
+            return
+        messages = result.get("task_messages") or []
+        if not messages:
+            return
+        delivered = 0
+        rejected = []
+        for msg in messages:
+            try:
+                conn = _db_connect(self.db_path)
+                try:
+                    status, detail = _deliver_peer_message(
+                        conn, task["id"], msg["to_task_id"],
+                        msg["body"], msg["mode"])
+                finally:
+                    conn.close()
+            except Exception as exc:
+                status, detail = "rejected", "delivery error: %s" % exc
+            if status in ("delivered", "revived"):
+                delivered += 1
+            else:
+                rejected.append("%s: %s" % (msg["to_task_id"], detail))
+        detail = "%s: delivered=%d rejected=%d" % (
+            task["id"], delivered, len(rejected))
+        if rejected:
+            detail += " (" + "; ".join(rejected) + ")"
+        self._audit(task["project_id"], "worker_peer_messages", detail)
+
     def _run_attempt(self, task, quota_check=None):
         """单次执行尝试（不含认领、key 门、重试、成本与最终审计）。
 
@@ -2121,6 +2165,7 @@ class Worker:
             result, error = self._run_fork(task, quota_check=_quota_hit)
             usage_total = sum_usage(usage_total,
                                     (result or {}).get("usage"))
+            self._deliver_peer_messages(task, result)
             if (result or {}).get("status") == "quota_exceeded":
                 quota_error = self._quota_exceeded(task, usage_total) or \
                     "quota_exceeded: 本月配额已用完"
@@ -2137,6 +2182,7 @@ class Worker:
                     break
             result, error = self._run_attempt(task, quota_check=_quota_hit)
             usage_total = sum_usage(usage_total, (result or {}).get("usage"))
+            self._deliver_peer_messages(task, result)
             if (result or {}).get("status") == "quota_exceeded":
                 # 轮询掐断：进程已被杀，配额原因以现查为准。
                 quota_error = self._quota_exceeded(task, usage_total) or \
@@ -2188,6 +2234,7 @@ class Worker:
                 result, error = self._run_attempt(task, quota_check=_quota_hit)
                 usage_total = sum_usage(usage_total,
                                         (result or {}).get("usage"))
+                self._deliver_peer_messages(task, result)
                 if (result or {}).get("status") == "quota_exceeded":
                     quota_error = self._quota_exceeded(task, usage_total) or \
                         "quota_exceeded: 本月配额已用完"
@@ -2239,11 +2286,17 @@ class Worker:
                     quota_stop = True
                     break
                 fu_task = dict(task)
-                fu_task["prompt"] = fu["body"]
+                # 任务间消息的接收方提示：带发送方 id，接收方
+                # agent 凭此知道回给哪个任务；用户后续消息无前缀。
+                fu_prompt = fu["body"]
+                if fu.get("sender_task_id"):
+                    fu_prompt = _peer_prefix(fu["sender_task_id"]) + fu_prompt
+                fu_task["prompt"] = fu_prompt
                 fu_result, fu_error = self._run_attempt(
                     fu_task, quota_check=_quota_hit)
                 usage_total = sum_usage(
                     usage_total, (fu_result or {}).get("usage"))
+                self._deliver_peer_messages(task, fu_result)
                 if (fu_result or {}).get("status") == "quota_exceeded":
                     self._restore_followup(fu["id"])
                     quota_error = self._quota_exceeded(task, usage_total) or \