feat(governance): auto-deprecate low-trust unused playbooks (>30d)

trust_drift previously fired alerts forever for playbooks stuck below
the 0.2 threshold. With user authorization for governance-class
auto-fixes, check_trust_drift now retires playbooks that have been
unused for 30+ days (or never used and created 30+ days ago) by
flipping status to 'deprecated' before alerting.

Alerts now report drifted_count, auto_deprecated_count, and the kept
playbook_ids that still need human review (those in their 30d trial
window). Existing alert noise from the four currently-drifted
playbooks should drop to whatever fraction is genuinely in trial.

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

apps/api/src/services/governance_agent.py

@@ -39,6 +39,10 @@ logger = structlog.get_logger(__name__)
 # 閾值常數
 # =============================================================================
 TRUST_DRIFT_THRESHOLD = 0.2          # playbook trust_score 低於此值 → 告警
+# 2026-05-02 ogt + Claude Sonnet 4.6: trust_drift auto-deprecate
+# trust < 0.2 + (last_used > N 天前 OR 從沒用過 + 創建超過 N 天) → 自動 deprecate
+# 這個 N 設 30 天，給 playbook 充足試用期，避免新提案被早期幾次失敗就廢棄
+TRUST_DRIFT_AUTO_DEPRECATE_AFTER_DAYS = 30
 KM_STALE_DAYS = 7                     # 知識條目超過幾天未更新視為陳舊
 KM_STALE_RATIO = 0.20                 # 陳舊比例超過此值 → 告警
 HALLUCINATION_RATE_THRESHOLD = 0.10   # LLM verification failed 比例超過此值 → 告警
@@ -69,9 +73,12 @@ class GovernanceAgent:
     # =========================================================================
 
     async def check_trust_drift(self) -> dict[str, Any]:
-        """Playbook trust_score < 0.2 → 告警建議廢棄
+        """Playbook trust_score < 0.2 → 告警建議廢棄；30 天沒用過的直接 auto-deprecate
 
         2026-04-26 P2.2 by Claude
+        2026-05-02 ogt + Claude Sonnet 4.6: 加 auto_deprecate_low_trust_unused 自治路徑
+        守衛條件：trust < 0.2 AND (last_used_at < 30 天前 OR 從未使用且創建超過 30 天)
+        → status 改 'deprecated'，alert 改報「N 個告警 + M 個 auto-deprecated」
         """
         async with get_db_context() as db:
             result = await db.execute(
@@ -81,9 +88,31 @@ class GovernanceAgent:
             )
             all_records = result.scalars().all()
 
-        total = len(all_records)
-        drifted = [r for r in all_records if float(r.trust_score) < TRUST_DRIFT_THRESHOLD]
-        drifted_ids = [r.playbook_id for r in drifted[:10]]
+            total = len(all_records)
+            drifted = [r for r in all_records if float(r.trust_score) < TRUST_DRIFT_THRESHOLD]
+
+            # auto-deprecate eligibility
+            cutoff = now_taipei() - timedelta(days=TRUST_DRIFT_AUTO_DEPRECATE_AFTER_DAYS)
+            auto_deprecated_ids: list[str] = []
+            kept_ids: list[str] = []
+            for r in drifted:
+                last = r.last_used_at
+                created = r.created_at
+                # 沒用過 → 用 created_at 作為「進入系統時間」
+                ref_time = last if last is not None else created
+                if ref_time is not None and ref_time < cutoff:
+                    r.status = "deprecated"
+                    auto_deprecated_ids.append(r.playbook_id)
+                else:
+                    kept_ids.append(r.playbook_id)
+
+            if auto_deprecated_ids:
+                await db.commit()
+                logger.info(
+                    "governance_trust_drift_auto_deprecated",
+                    count=len(auto_deprecated_ids),
+                    ids=auto_deprecated_ids[:10],
+                )
 
         if drifted:
             await self._alert(
@@ -91,8 +120,11 @@ class GovernanceAgent:
                 {
                     "drifted_count": len(drifted),
                     "total_playbooks": total,
-                    "playbook_ids": drifted_ids,
+                    "playbook_ids": kept_ids[:10],
+                    "auto_deprecated_count": len(auto_deprecated_ids),
+                    "auto_deprecated_ids": auto_deprecated_ids[:10],
                     "threshold": TRUST_DRIFT_THRESHOLD,
+                    "auto_deprecate_after_days": TRUST_DRIFT_AUTO_DEPRECATE_AFTER_DAYS,
                 },
             )
 
@@ -100,8 +132,15 @@ class GovernanceAgent:
             "governance_trust_drift_checked",
             total=total,
             drifted=len(drifted),
+            auto_deprecated=len(auto_deprecated_ids),
+            kept=len(kept_ids),
         )
-        return {"checked": total, "drifted": len(drifted)}
+        return {
+            "checked": total,
+            "drifted": len(drifted),
+            "auto_deprecated": len(auto_deprecated_ids),
+            "kept": len(kept_ids),
+        }
 
     # =========================================================================
     # 2. 知識庫衰退