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/tests/unit/test_task_execution_planner.py

@@ -0,0 +1,156 @@
+from __future__ import annotations
+
+import asyncio
+from types import SimpleNamespace
+
+from beaver.engine.providers.base import LLMProvider, LLMResponse
+from beaver.engine.providers.factory import ProviderBundle
+from beaver.tasks import TaskExecutionPlanner, TaskRecord
+
+
+class PlannerProvider(LLMProvider):
+    def __init__(self, response: str) -> None:
+        super().__init__()
+        self.response = response
+
+    async def chat(
+        self,
+        messages: list[dict],
+        tools: list[dict] | None = None,
+        model: str | None = None,
+        max_tokens: int = 4096,
+        temperature: float = 0.7,
+    ) -> LLMResponse:
+        return LLMResponse(content=self.response, finish_reason="stop", provider_name="stub", model="stub-model")
+
+    def get_default_model(self) -> str:
+        return "stub-model"
+
+
+def _task() -> TaskRecord:
+    return TaskRecord(
+        task_id="task-1",
+        session_id="session-1",
+        description="implement workflow",
+        goal="implement workflow",
+        constraints=[],
+        priority=0,
+        status="open",
+        creator="test",
+        created_at="now",
+        updated_at="now",
+    )
+
+
+def _bundle(response: str) -> ProviderBundle:
+    return ProviderBundle(
+        main_runtime=SimpleNamespace(model="stub-model", provider_name="stub"),
+        main_provider=PlannerProvider(response),
+    )
+
+
+def test_planner_selects_single_mode() -> None:
+    plan = asyncio.run(
+        TaskExecutionPlanner().plan(
+            task=_task(),
+            user_message="implement workflow",
+            attempt_index=1,
+            provider_bundle=_bundle('{"mode":"single","reason":"main agent is enough"}'),
+        )
+    )
+
+    assert plan.mode == "single"
+    assert plan.graph is None
+    assert plan.reason == "main agent is enough"
+
+
+def test_planner_builds_team_graph() -> None:
+    plan = asyncio.run(
+        TaskExecutionPlanner().plan(
+            task=_task(),
+            user_message="implement workflow",
+            attempt_index=1,
+            provider_bundle=_bundle(
+                """
+                {
+                  "mode": "team",
+                  "reason": "needs parallel review",
+                  "strategy": "dag",
+                  "nodes": [
+                    {"node_id": "research", "task": "research options", "agent": {"name": "researcher"}},
+                    {"node_id": "review", "task": "review result", "agent": {"name": "reviewer"}, "depends_on": ["research"]}
+                  ],
+                  "final_synthesis_instruction": "merge the findings"
+                }
+                """
+            ),
+        )
+    )
+
+    assert plan.is_team
+    assert plan.graph is not None
+    assert plan.graph.strategy == "dag"
+    assert [node.node_id for node in plan.graph.nodes] == ["research", "review"]
+    assert plan.graph.nodes[1].depends_on == ["research"]
+    assert plan.final_synthesis_instruction == "merge the findings"
+
+
+def test_planner_team_nodes_can_target_skills_without_agent_roles() -> None:
+    plan = TaskExecutionPlanner().from_json(
+        """
+        {
+          "mode": "team",
+          "reason": "needs skill-guided review",
+          "strategy": "sequence",
+          "nodes": [
+            {
+              "node_id": "api_review",
+              "task": "review API compatibility",
+              "skill_query": "API contract compatibility review",
+              "required_capabilities": ["schema compatibility"]
+            }
+          ]
+        }
+        """
+    )
+
+    assert plan.is_team
+    assert plan.graph is not None
+    node = plan.graph.nodes[0]
+    assert node.agent.name == "api_review"
+    assert node.agent.role == ""
+    assert node.agent.metadata["skill_query"] == "API contract compatibility review"
+    assert node.agent.metadata["required_capabilities"] == ["schema compatibility"]
+
+
+def test_planner_invalid_outputs_fallback_to_single() -> None:
+    planner = TaskExecutionPlanner()
+    invalid_json = planner.from_json("not json")
+    unknown_strategy = planner.from_json(
+        '{"mode":"team","strategy":"moa","nodes":[{"node_id":"a","task":"a","agent":{"name":"a"}}]}'
+    )
+    too_many_nodes = planner.from_json(
+        '{"mode":"team","strategy":"parallel","nodes":['
+        + ",".join(
+            '{"node_id":"n%s","task":"work","agent":{"name":"n%s"}}' % (index, index)
+            for index in range(7)
+        )
+        + "]}"
+    )
+    cyclic = planner.from_json(
+        """
+        {
+          "mode": "team",
+          "strategy": "dag",
+          "nodes": [
+            {"node_id": "a", "task": "a", "agent": {"name": "a"}, "depends_on": ["b"]},
+            {"node_id": "b", "task": "b", "agent": {"name": "b"}, "depends_on": ["a"]}
+          ]
+        }
+        """
+    )
+
+    assert invalid_json.mode == "single"
+    assert unknown_strategy.mode == "single"
+    assert too_many_nodes.mode == "single"
+    assert cyclic.mode == "single"