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/skills/learning/__init__.py

@@ -0,0 +1,24 @@
+"""Skill learning loop helpers."""
+
+from .evidence import EvidencePacket, EvidenceSelector
+from .eval import SkillDraftEvaluator
+from .missing_skill import MissingSkillDraftResult, MissingSkillSynthesizer
+from .pipeline import SkillLearningPipelineService
+from .service import RunReceiptContext, SkillLearningService
+from .synthesizer import SkillDraftSynthesizer
+from .worker import SkillLearningWorker, SkillLearningWorkerConfig, SkillLearningWorkerResult
+
+__all__ = [
+    "EvidencePacket",
+    "EvidenceSelector",
+    "SkillDraftEvaluator",
+    "MissingSkillDraftResult",
+    "MissingSkillSynthesizer",
+    "RunReceiptContext",
+    "SkillLearningPipelineService",
+    "SkillDraftSynthesizer",
+    "SkillLearningService",
+    "SkillLearningWorker",
+    "SkillLearningWorkerConfig",
+    "SkillLearningWorkerResult",
+]