fix(aiops): 修復 evidence 空白 → AI ABSTAIN 問題

問題：
- signal.alert_name 在頂層，但 _get_alertname() 從 labels["alertname"] 讀 → 空字串
- 所有 sensor 失敗時 evidence_summary 只有 120 字元，AI 無法分析 → ABSTAIN
- labels 為空時 AI 根本不知道是什麼告警

修復：
1. _get_alertname(): 優先讀 signal.alert_name，fallback labels["alertname"]
2. _get_labels(): 自動補 alertname 到 labels dict
3. EvidenceSnapshot.alert_info: 新增告警基礎欄位（sensors=0 時的最小情報）
4. build_summary(): alert_info 永遠放在最前，讓 AI 至少知道告警類型+嚴重度

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

apps/api/src/services/evidence_snapshot.py

@@ -77,6 +77,9 @@ class EvidenceSnapshot:
     schema_version: str = SCHEMA_VERSION
     collected_at: datetime = field(default_factory=now_taipei)
 
+    # 告警基礎資訊（sensors=0 時的最小情報，2026-04-16 ogt + Claude Sonnet 4.6）
+    alert_info: dict[str, Any] | None = None
+
     # 8D 感官數據
     k8s_state: dict[str, Any] | None = None          # D1
     recent_logs: str | None = None                   # D2 (sanitized)
@@ -134,6 +137,10 @@ class EvidenceSnapshot:
         """
         parts: list[str] = []
 
+        # 告警基礎資訊永遠放在最前（sensors=0 時也要讓 AI 知道是什麼告警）
+        if self.alert_info:
+            parts.append(f"[告警資訊] {self.alert_info}")
+
         if self.k8s_state:
             parts.append(f"[K8s狀態] {self.k8s_state}")
         if self.recent_logs:

apps/api/src/services/pre_decision_investigator.py

@@ -106,6 +106,22 @@ class PreDecisionInvestigator:
         snapshot = EvidenceSnapshot(incident_id=incident_id)
         snapshot.sensors_attempted = len(tools)
 
+        # 告警基礎資訊：sensors=0 時 AI 至少知道是什麼告警
+        # 2026-04-16 ogt + Claude Sonnet 4.6: 修復空 evidence → ABSTAIN 問題
+        sigs = getattr(incident, "signals", []) or []
+        sig0 = sigs[0] if sigs else None
+        snapshot.alert_info = {
+            "alert_name": alertname or getattr(incident, "alertname", "") or "",
+            "severity": str(getattr(incident, "severity", "")),
+            "affected_services": getattr(incident, "affected_services", []) or [],
+            "labels": labels,
+            "annotations": (
+                ({k: v for k, v in (sig0.annotations or {}).items()} if sig0 else {})
+            ),
+            "source": getattr(sig0, "source", "") if sig0 else "",
+            "incident_id": incident_id,
+        }
+
         # 3. 並行蒐集（整體 INVESTIGATOR_TIMEOUT_SEC 保護）
         try:
             await asyncio.wait_for(
@@ -343,13 +359,26 @@ def _fill_snapshot_dimension(
 
 def _get_alertname(incident: "Incident") -> str:
     if incident.signals:
-        return incident.signals[0].labels.get("alertname", "")
-    return ""
+        sig = incident.signals[0]
+        # alert_name 在 Signal 頂層欄位，labels["alertname"] 是 Prometheus 慣例但可能為空
+        return (
+            getattr(sig, "alert_name", "")
+            or sig.labels.get("alertname", "")
+            or getattr(incident, "alertname", "")
+            or ""
+        )
+    return getattr(incident, "alertname", "") or ""
 
 
 def _get_labels(incident: "Incident") -> dict[str, Any]:
     if incident.signals:
-        return incident.signals[0].labels
+        sig = incident.signals[0]
+        labels = sig.labels or {}
+        # 若 labels 缺少 alertname，補上頂層的 alert_name
+        if "alertname" not in labels and getattr(sig, "alert_name", ""):
+            labels = dict(labels)
+            labels["alertname"] = sig.alert_name
+        return labels
     return {}