```

feat(learning): 添加技能学习候选者合成锁定机制

添加了 DraftSynthesisInProgress 和 DraftHasNoChanges 异常来处理并发场景，
确保同一技能学习候选者的合成过程不会重复执行。实现了 claim_learning_candidate_for_synthesis
方法来原子性地锁定候选者进行合成。

fix(web): 为技能草案创建端点添加适当的HTTP状态码

当草案没有变化或正在合成时，现在正确返回409状态码而不是内部错误。

feat(skills): 实现技能修订内容比较以检测无变化情况

添加了 _is_noop_revision 方法来比较基础技能和提议的修订，
如果内容没有实际变化则抛出 NoDraftChanges 异常。

refactor(process): 修复任务证据记录后根运行状态更新逻辑

将任务证据记录事件后的状态从 waiting 更改为 done，并设置 finished_at 时间戳。

feat(tools): 防止在同一运行中重复执行外部写入操作

为邮件发送、日历创建等外部写入工具添加去重机制，避免重复的外部操作。

test: 添加技能学习和工具执行的单元测试

增加测试用例验证并发草案合成、重复外部写入抑制和无变化修订检测等功能。
```

app-instance/backend/beaver/skills/learning/pipeline.py

@@ -9,7 +9,7 @@ from beaver.memory.skills import SkillDraftEvalReport, SkillDraftSafetyReport, S
 from beaver.skills.drafts import DraftService
 from beaver.skills.learning.eval import SkillDraftEvaluator
 from beaver.skills.learning.replay import ReplayRunner
-from beaver.skills.learning.service import SkillLearningService
+from beaver.skills.learning.service import NoDraftChanges, SkillLearningService
 from beaver.skills.learning.safety import SkillDraftSafetyChecker
 from beaver.skills.publisher import SkillPublisher
 from beaver.skills.reviews import ReviewService
@@ -22,6 +22,14 @@ _REJECTABLE_DRAFT_STATUSES = {
 }
 
 
+class DraftSynthesisInProgress(RuntimeError):
+    """Raised when another request already claimed the candidate for synthesis."""
+
+
+class DraftHasNoChanges(RuntimeError):
+    """Raised when synthesis produced no effective changes from the base skill."""
+
+
 class SkillLearningPipelineService:
     """Coordinates candidate -> draft -> review -> publish lifecycle."""
 
@@ -60,8 +68,23 @@ class SkillLearningPipelineService:
         candidate_id: str,
         *,
         provider_bundle: ProviderBundle,
+        force: bool = False,
     ) -> SkillDraft:
-        draft = await self.learning_service.synthesize_draft(candidate_id, provider_bundle)
+        if not force:
+            existing = self._draft_for_candidate(candidate_id)
+            if existing is not None:
+                return existing
+        claimed = self.learning_store.claim_learning_candidate_for_synthesis(candidate_id, force=force)
+        if claimed is None:
+            existing = self._draft_for_candidate(candidate_id)
+            if existing is not None:
+                return existing
+            raise DraftSynthesisInProgress(f"Draft synthesis is already in progress for candidate: {candidate_id}")
+        try:
+            draft = await self.learning_service.synthesize_draft(candidate_id, provider_bundle)
+        except NoDraftChanges as exc:
+            self.mark_candidate_superseded(candidate_id, str(exc))
+            raise DraftHasNoChanges(str(exc)) from exc
         self.mark_draft_synthesized(candidate_id, draft)
         return draft
 
@@ -71,13 +94,7 @@ class SkillLearningPipelineService:
         *,
         provider_bundle: ProviderBundle,
     ) -> SkillDraft:
-        self.learning_store.transition_learning_candidate(
-            candidate_id,
-            "synthesizing",
-            event_type="draft_synthesis_started",
-            last_error=None,
-        )
-        return await self.synthesize_draft(candidate_id, provider_bundle=provider_bundle)
+        return await self.synthesize_draft(candidate_id, provider_bundle=provider_bundle, force=True)
 
     def mark_candidate_queued(self, candidate_id: str) -> SkillLearningCandidate:
         return self._require_updated(
@@ -162,6 +179,12 @@ class SkillLearningPipelineService:
             raise ValueError(f"Draft not found: {skill_name}/{draft_id}")
         return draft
 
+    def _draft_for_candidate(self, candidate_id: str) -> SkillDraft | None:
+        candidate = self.get_candidate(candidate_id)
+        if not candidate.draft_skill_name or not candidate.draft_id:
+            return None
+        return self.draft_service.get_draft(candidate.draft_skill_name, candidate.draft_id)
+
     def submit_review(
         self,
         skill_name: str,