feat(sre): add guarded HolmesGPT shadow investigator

apps/api/src/core/config.py

@@ -1180,6 +1180,40 @@ class Settings(BaseSettings):
         description="P3.1-T2-PathA: 啟用 DiagnosisAggregator 信號分類層補 PDI（路徑 A：不重複收集，只分類已有 raw 資料）",
     )
 
+    # AIA-SRE-012: HolmesGPT 只讀 shadow investigator。
+    # 必須同時提供 internal allowlist、modelList alias 與 immutable artifact digest；
+    # 任一缺失都 fail closed，且不會取得 executor/runtime write authority。
+    ENABLE_HOLMESGPT_SHADOW: bool = Field(
+        default=False,
+        description="Enable the read-only HolmesGPT shadow investigator",
+    )
+    HOLMESGPT_BASE_URL: str = Field(
+        default="",
+        description="Allowlisted internal HolmesGPT base URL; never exposed in receipts",
+    )
+    HOLMESGPT_ALLOWED_ORIGINS: str = Field(
+        default="",
+        description="Comma-separated exact origin allowlist for the internal HolmesGPT endpoint",
+    )
+    HOLMESGPT_MODEL_ALIAS: str = Field(
+        default="",
+        description="HolmesGPT modelList alias, not a direct provider model identifier",
+    )
+    HOLMESGPT_ARTIFACT_DIGEST: str = Field(
+        default="",
+        description="Pinned internal HolmesGPT artifact digest in sha256:<64 hex> form",
+    )
+    HOLMESGPT_API_KEY: str = Field(
+        default="",
+        description="Protected HolmesGPT API key; never logged or included in receipts",
+    )
+    HOLMESGPT_SHADOW_TIMEOUT_SECONDS: float = Field(
+        default=3.0,
+        ge=0.5,
+        le=5.0,
+        description="Bounded HolmesGPT shadow request timeout",
+    )
+
     # ==========================================================================
     # W2 PR-V1: SelfHealingValidator Feature Flag (2026-04-28 ogt + Claude Sonnet 4.6)
     # 飛輪斷鏈 C6 修復 — 驗證層串接自愈品質評估