```

feat(learning): 修复任务运行记录排序逻辑处理空attempt_index的情况

当RunRecord的attempt_index为None时，之前的排序逻辑会出现问题。
现在通过在排序键中显式处理None值来解决这个问题，
将None值排在前面，并将其转换为0进行比较。

同时添加了单元测试验证团队运行记录（没有attempt_index）的处理情况。
```

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

@@ -462,7 +462,15 @@ class SkillLearningService:
 
     @staticmethod
     def _representative_task_text(runs: list[RunRecord], *, fallback: str = "") -> str:
-        ordered = sorted(runs, key=lambda item: (item.attempt_index, item.started_at, item.run_id))
+        ordered = sorted(
+            runs,
+            key=lambda item: (
+                item.attempt_index is None,
+                item.attempt_index if item.attempt_index is not None else 0,
+                item.started_at,
+                item.run_id,
+            ),
+        )
         for record in ordered:
             text = record.task_text.strip()
             if text:

app-instance/backend/tests/unit/test_phase5_skills_runtime.py

@@ -591,6 +591,51 @@ def test_skill_learning_service_uses_original_task_text_for_new_skill_theme(tmp_
     assert candidates[0].evidence["task_text"] == "Compare direct production restart with staging rollout"
 
 
+def test_skill_learning_service_handles_team_runs_without_attempt_index(tmp_path: Path) -> None:
+    store = SkillSpecStore(tmp_path)
+    run_store = RunMemoryStore(tmp_path / "memory" / "runs")
+    learning_store = SkillLearningStore(tmp_path / "memory" / "skills")
+    service = SkillLearningService(
+        run_store=run_store,
+        learning_store=learning_store,
+        draft_service=DraftService(store),
+        evidence_selector=EvidenceSelector(run_store),
+    )
+    now = datetime.now(timezone.utc).isoformat()
+    run_store.append_run_record(
+        RunRecord(
+            run_id="team-run",
+            session_id="session-task:team:research",
+            task_id="task-1",
+            attempt_index=None,
+            task_text="Research one product",
+            started_at=now,
+            ended_at=now,
+            success=True,
+            finish_reason="stop",
+        )
+    )
+    run_store.append_run_record(
+        RunRecord(
+            run_id="main-run",
+            session_id="session-task",
+            task_id="task-1",
+            attempt_index=1,
+            task_text="Compare two products and email the report",
+            started_at=now,
+            ended_at=now,
+            success=True,
+            finish_reason="stop",
+            feedback={"acceptance_type": "accept"},
+        )
+    )
+
+    candidates = service.build_learning_candidates_for_task("task-1", final_accepted_run_id="main-run")
+
+    assert [candidate.candidate_id for candidate in candidates] == ["new:task:task-1"]
+    assert candidates[0].evidence["task_text"] == "Compare two products and email the report"
+
+
 def test_task_theme_uses_first_sentence_for_chinese_text() -> None:
     assert (
         SkillLearningService._task_theme(