feat(beaver): 完成Task Team功能v1实现，重构后端架构支持统一内核

新增内部Task系统，包括验证、反馈门控机制，实现自动质量验证
(通过率>=0.75)和用户反馈闭环(satisfied/revise/abandon)。

实现Agent Team v1协调器，支持sequence/parallel/dag执行策略，
sub-agent复用主AgentLoop，每个run使用独立memory snapshot。

建立Skill学习pipeline，包含draft/审核/发布/回滚完整生命周期，
通过Task验证通过且用户满意才生成学习候选。

重构目录结构，移除third_party依赖，建立统一engine内核，
所有agent共享运行时基础组件。

更新ContextBuilder清理provider消息字段，增强SkillContext版本管理，
集成TaskExecutionPlanner和TaskSkillResolver实现技能解析机制。

app-instance/backend/beaver/tasks/router.py

@@ -0,0 +1,40 @@
+"""Main Agent routing between simple chat and internal Task mode."""
+
+from __future__ import annotations
+
+import re
+
+from .models import MainAgentDecision, TaskRecord
+
+
+class MainAgentRouter:
+    """Small deterministic classifier used before the main AgentLoop.
+
+    The first version intentionally avoids a mandatory model call so the router
+    stays reliable during provider outages. The rule set is conservative:
+    anything that implies execution, files, tools, iteration, or validation
+    becomes Task mode.
+    """
+
+    _TASK_PATTERNS = [
+        r"\b(implement|fix|debug|refactor|migrate|build|create|write|edit|update|test|validate|deploy)\b",
+        r"\b(file|repo|code|project|backend|frontend|api|database|migration|pull request|ci|bug)\b",
+        r"\b(step|multi-step|workflow|plan and|then)\b",
+        r"(实现|修复|调试|重构|迁移|构建|创建|编写|修改|更新|测试|验证|部署|文件|代码|项目|前端|后端|接口|数据库|多步|任务)",
+    ]
+    _NEW_TASK_PATTERNS = [
+        r"\b(new task|another task|different task|start over)\b",
+        r"(新任务|另一个任务|换个任务|重新开始)",
+    ]
+
+    def classify(self, message: str, *, active_task: TaskRecord | None = None) -> MainAgentDecision:
+        text = message.strip()
+        lowered = text.lower()
+        starts_new = any(re.search(pattern, lowered, re.IGNORECASE) for pattern in self._NEW_TASK_PATTERNS)
+        if active_task is not None and active_task.status in {"awaiting_feedback", "needs_revision"} and not starts_new:
+            return MainAgentDecision(mode="task", reason="continuing_open_task", starts_new_task=False)
+        if any(re.search(pattern, lowered, re.IGNORECASE) for pattern in self._TASK_PATTERNS):
+            return MainAgentDecision(mode="task", reason="task_pattern_matched", starts_new_task=starts_new)
+        if len(text) > 240:
+            return MainAgentDecision(mode="task", reason="long_request", starts_new_task=starts_new)
+        return MainAgentDecision(mode="simple", reason="simple_question", starts_new_task=False)