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/drafts/__init__.py

@@ -1,2 +1,6 @@
 """Draft skills generated before review."""
+"""Skill draft services."""
 
+from .service import DraftService
+
+__all__ = ["DraftService"]

app-instance/backend/beaver/skills/drafts/service.py

@@ -0,0 +1,131 @@
+"""Draft lifecycle for Beaver skills."""
+
+from __future__ import annotations
+
+from uuid import uuid4
+
+from beaver.skills.specs import SkillDraft, SkillSpecStore
+
+
+class DraftService:
+    def __init__(self, store: SkillSpecStore) -> None:
+        self.store = store
+
+    def create_new_skill_draft(
+        self,
+        *,
+        skill_name: str,
+        proposed_content: str,
+        proposed_frontmatter: dict,
+        created_by: str,
+        reason: str,
+        trigger_run_id: str | None = None,
+        trigger_session_id: str | None = None,
+        evidence_refs: list[dict] | None = None,
+    ) -> SkillDraft:
+        draft = SkillDraft(
+            draft_id=uuid4().hex,
+            skill_name=skill_name,
+            base_version=None,
+            proposed_content=proposed_content,
+            proposed_frontmatter=dict(proposed_frontmatter),
+            created_at=_utc_now(),
+            created_by=created_by,
+            trigger_run_id=trigger_run_id,
+            trigger_session_id=trigger_session_id,
+            reason=reason,
+            evidence_refs=list(evidence_refs or []),
+            proposal_kind="new_skill",
+        )
+        self.store.write_draft(draft)
+        return draft
+
+    def create_revision_draft(
+        self,
+        *,
+        skill_name: str,
+        base_version: str | None,
+        proposed_content: str,
+        proposed_frontmatter: dict,
+        created_by: str,
+        reason: str,
+        trigger_run_id: str | None = None,
+        trigger_session_id: str | None = None,
+        evidence_refs: list[dict] | None = None,
+    ) -> SkillDraft:
+        draft = SkillDraft(
+            draft_id=uuid4().hex,
+            skill_name=skill_name,
+            base_version=base_version,
+            proposed_content=proposed_content,
+            proposed_frontmatter=dict(proposed_frontmatter),
+            created_at=_utc_now(),
+            created_by=created_by,
+            trigger_run_id=trigger_run_id,
+            trigger_session_id=trigger_session_id,
+            reason=reason,
+            evidence_refs=list(evidence_refs or []),
+            proposal_kind="revise_skill",
+        )
+        self.store.write_draft(draft)
+        return draft
+
+    def create_merge_draft(
+        self,
+        *,
+        skill_name: str,
+        base_version: str | None,
+        proposed_content: str,
+        proposed_frontmatter: dict,
+        created_by: str,
+        reason: str,
+        evidence_refs: list[dict] | None = None,
+    ) -> SkillDraft:
+        draft = self.create_revision_draft(
+            skill_name=skill_name,
+            base_version=base_version,
+            proposed_content=proposed_content,
+            proposed_frontmatter=proposed_frontmatter,
+            created_by=created_by,
+            reason=reason,
+            evidence_refs=evidence_refs,
+        )
+        draft.proposal_kind = "merge_skills"
+        self.store.write_draft(draft)
+        return draft
+
+    def create_retire_proposal(
+        self,
+        *,
+        skill_name: str,
+        base_version: str | None,
+        created_by: str,
+        reason: str,
+        evidence_refs: list[dict] | None = None,
+    ) -> SkillDraft:
+        draft = SkillDraft(
+            draft_id=uuid4().hex,
+            skill_name=skill_name,
+            base_version=base_version,
+            proposed_content="",
+            proposed_frontmatter={},
+            created_at=_utc_now(),
+            created_by=created_by,
+            reason=reason,
+            evidence_refs=list(evidence_refs or []),
+            proposal_kind="retire_skill",
+        )
+        self.store.write_draft(draft)
+        return draft
+
+    def list_drafts(self, skill_name: str | None = None) -> list[SkillDraft]:
+        return self.store.list_drafts(skill_name)
+
+    def get_draft(self, skill_name: str, draft_id: str) -> SkillDraft | None:
+        return self.store.read_draft(skill_name, draft_id)
+
+
+def _utc_now() -> str:
+    from datetime import datetime, timezone
+
+    return datetime.now(timezone.utc).isoformat()