feat: add Main Agent core service, tools, and Celery tasks (Phase 2)

数字员工大脑 — Main Agent 核心实现：
- MainAgentService: 目标分解（LLM）、任务调度、进度监控、失败重试、自主循环
- 4个 Main Agent 专有工具: create_task / assign_task / check_progress / notify_user
- Celery 异步任务: decompose_goal / execute_goal / execute_task / autonomy_tick
- Goal API 增强: decompose / execute-async / replan 端点

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/tasks/goal_tasks.py

@@ -0,0 +1,132 @@
+"""
+Goal/Task 异步任务 — Celery 任务定义
+
+Main Agent 的目标分解、任务执行、自主循环等重量级操作通过 Celery 异步执行。
+"""
+from app.core.tools_bootstrap import ensure_builtin_tools_registered
+
+ensure_builtin_tools_registered()
+
+from app.core.celery_app import celery_app
+from app.core.database import SessionLocal
+from app.models.goal import Goal
+from app.models.execution import Execution
+from app.services.main_agent_service import MainAgentService
+import asyncio
+import logging
+import time
+
+logger = logging.getLogger(__name__)
+
+
+@celery_app.task(bind=True)
+def decompose_goal_task(self, goal_id: str):
+    """
+    异步分解目标：使用 LLM 将 Goal 分解为 Task 树。
+
+    在 Celery Worker 中执行，避免 API 请求超时。
+    """
+    db = SessionLocal()
+    try:
+        service = MainAgentService(db)
+        goal = asyncio.run(service.decompose_goal(goal_id))
+
+        return {
+            "status": "completed",
+            "goal_id": goal_id,
+            "goal_title": goal.title,
+        }
+    except Exception as e:
+        logger.error(f"Goal decomposition failed: {e}", exc_info=True)
+        db.rollback()
+        return {"status": "failed", "goal_id": goal_id, "error": str(e)}
+    finally:
+        db.close()
+
+
+@celery_app.task(bind=True)
+def execute_goal_task(self, goal_id: str):
+    """
+    异步执行 Goal：启动 Main Agent 管理目标全生命周期。
+
+    1. 分解目标（如果尚未分解）
+    2. 持续执行 task 直到完成或阻塞
+    3. 更新 Goal 状态
+    """
+    db = SessionLocal()
+    start_time = time.time()
+    try:
+        goal = db.query(Goal).filter(Goal.id == goal_id).first()
+        if not goal:
+            return {"status": "failed", "goal_id": goal_id, "error": "目标不存在"}
+
+        # 更新状态
+        goal.status = "active"
+        db.commit()
+
+        service = MainAgentService(db)
+        result = asyncio.run(service.start_goal_execution(goal_id))
+
+        elapsed = int((time.time() - start_time) * 1000)
+        return {
+            "status": "completed",
+            "goal_id": goal_id,
+            "elapsed_ms": elapsed,
+            **result,
+        }
+    except Exception as e:
+        logger.error(f"Goal execution failed: {e}", exc_info=True)
+        goal = db.query(Goal).filter(Goal.id == goal_id).first()
+        if goal:
+            goal.status = "failed"
+            db.commit()
+        return {"status": "failed", "goal_id": goal_id, "error": str(e)}
+    finally:
+        db.close()
+
+
+@celery_app.task(bind=True)
+def execute_task_celery(self, task_id: str):
+    """
+    异步执行单个 Task。
+
+    Main Agent 创建 Execution 记录后将任务交给 Celery Worker 执行。
+    """
+    db = SessionLocal()
+    start_time = time.time()
+    try:
+        service = MainAgentService(db)
+        result = asyncio.run(service.execute_task(task_id))
+
+        elapsed = int((time.time() - start_time) * 1000)
+        return {
+            "status": result.get("status", "completed"),
+            "task_id": task_id,
+            "elapsed_ms": elapsed,
+            **result,
+        }
+    except Exception as e:
+        logger.error(f"Task execution failed: {e}", exc_info=True)
+        return {"status": "failed", "task_id": task_id, "error": str(e)}
+    finally:
+        db.close()
+
+
+@celery_app.task(bind=True)
+def autonomy_tick_task(self, goal_id: str):
+    """
+    自主循环单次心跳：检查进度 → 执行可运行任务 → 处理失败 → 通知。
+
+    由 Celery Beat 定时调度（根据 Goal.autonomy_config.check_interval_minutes）。
+    """
+    db = SessionLocal()
+    try:
+        service = MainAgentService(db)
+        result = asyncio.run(service.autonomy_tick(goal_id))
+        logger.info(f"Autonomy tick for goal {goal_id}: {result.get('status', 'unknown')}")
+        return {"status": "completed", "goal_id": goal_id, **result}
+    except Exception as e:
+        logger.error(f"Autonomy tick failed for goal {goal_id}: {e}", exc_info=True)
+        return {"status": "failed", "goal_id": goal_id, "error": str(e)}
+    finally:
+        db.close()