feat(Phase 3): 學習閉環重建 — 三根因修復 + 2x EWMA + Evolver Agent

ADR-083 Phase 3 學習閉環重建：

**三根因修復**
- approval_execution.py: fire-and-forget create_task → await asyncio.wait_for(timeout=30) × 2
  （成功路徑 L265 + 失敗路徑 L353，超時記錄 learning_trigger_timeout metric，主流程不 crash）
- models/approval.py: ApprovalRequestBase 新增 matched_playbook_id 欄位
- decision_manager.py: _auto_execute 建立 ApprovalRequest 時填充 matched_playbook_id
- learning_service.py: 雙路徑查找 _matched_pb_id（matched_playbook_id + metadata fallback）

**2x EWMA 負向強化**
- models/playbook.py: 新增 trust_score: float = 0.3（EWMA 動態信任度欄位）
- repositories/playbook_repository.py: update_stats 加 EWMA
  成功: trust = 0.9 × old + 0.1 × 1.0
  失敗: trust = 0.8 × old + 0.2 × 0.0（衰減速度 2x）
  trust < 0.1 → log warning，等 Evolver 封存

**Evolver Agent（新建）**
- services/playbook_evolver.py: 三功能全靜態規則
  1. 低信任封存: trust < 0.1 → DEPRECATED
  2. 休眠封存: 30d 未使用 AND trust < 0.5 → DEPRECATED
  3. 相似合併: 症狀 Jaccard > 0.9 → 保留高 trust，封存低 trust
  AIOPS_P3_EVOLVER_ENABLED=False 預設關閉

**文件**
- ADR-083 學習閉環重建
- MASTER §8 Phase 3 完工記錄

AIOPS_P3_ENABLED=False（預設），骨架就位等統帥批准開啟

Co-Authored-By: Claude Sonnet 4.6（亞太）<noreply@anthropic.com>

apps/api/src/repositories/playbook_repository.py

@@ -292,7 +292,15 @@ class PlaybookRepository:
         playbook_id: str,
         success: bool,
     ) -> bool:
-        """更新執行統計"""
+        """
+        更新執行統計 + EWMA 信任度
+
+        ADR-083 Phase 3: 負向 2x 強化 EWMA 公式
+        成功: trust_new = 0.9 * trust_old + 0.1 * 1.0
+        失敗: trust_new = 0.8 * trust_old + 0.2 * 0.0（衰減速度 2x）
+        trust < 0.1 → 記錄警告，由 Evolver Agent 封存
+        2026-04-15 ogt + Claude Sonnet 4.6（亞太）: Phase 3 EWMA 實裝
+        """
         try:
             playbook = await self.get_by_id(playbook_id)
             if not playbook:
@@ -300,17 +308,32 @@ class PlaybookRepository:
 
             if success:
                 playbook.success_count += 1
+                # 正向 EWMA：alpha=0.1，正向結果權重較小（保守更新）
+                playbook.trust_score = 0.9 * playbook.trust_score + 0.1 * 1.0
             else:
                 playbook.failure_count += 1
+                # 負向 EWMA：alpha=0.2，失敗懲罰 2x（快速衰退）
+                playbook.trust_score = 0.8 * playbook.trust_score + 0.2 * 0.0
 
+            # 邊界保護
+            playbook.trust_score = max(0.0, min(1.0, playbook.trust_score))
             playbook.last_used_at = now_taipei()
             await self.update(playbook)
 
+            if playbook.trust_score < 0.1:
+                logger.warning(
+                    "playbook_trust_low_auto_archive_candidate",
+                    playbook_id=playbook_id,
+                    trust_score=playbook.trust_score,
+                    hint="Evolver Agent 應將此 Playbook 封存",
+                )
+
             logger.info(
                 "playbook_stats_updated",
                 playbook_id=playbook_id,
                 success=success,
                 success_rate=playbook.success_rate,
+                trust_score=playbook.trust_score,
             )
             return True