```

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/memory/skills/store.py

@@ -114,6 +114,52 @@ class SkillLearningStore:
             )
         return updated
 
+    def claim_learning_candidate_for_synthesis(
+        self,
+        candidate_id: str,
+        *,
+        force: bool = False,
+    ) -> SkillLearningCandidate | None:
+        """Atomically claim a candidate before the expensive draft synthesis step."""
+
+        with self._locked():
+            candidates = self.list_learning_candidates()
+            claimed: SkillLearningCandidate | None = None
+            for index, candidate in enumerate(candidates):
+                if candidate.candidate_id != candidate_id:
+                    continue
+                if candidate.status in {"queued", "synthesizing"}:
+                    return None
+                if not force and candidate.draft_skill_name and candidate.draft_id:
+                    return None
+                payload = candidate.to_dict()
+                payload.update(
+                    {
+                        "status": "synthesizing",
+                        "last_error": None,
+                        "updated_at": _utc_now(),
+                    }
+                )
+                claimed = SkillLearningCandidate.from_dict(payload)
+                candidates[index] = claimed
+                break
+            if claimed is None:
+                return None
+            self.candidates_path.parent.mkdir(parents=True, exist_ok=True)
+            self.candidates_path.write_text(
+                "".join(
+                    json.dumps(candidate.to_dict(), ensure_ascii=False, sort_keys=True) + "\n"
+                    for candidate in candidates
+                ),
+                encoding="utf-8",
+            )
+            self.append_audit_event(
+                candidate_id,
+                "draft_synthesis_started",
+                {"status": "synthesizing", "force": force},
+            )
+            return claimed
+
     def list_learning_candidates(self, status: str | None = None) -> list[SkillLearningCandidate]:
         results: list[SkillLearningCandidate] = []
         for payload in self._read_jsonl(self.candidates_path):