feat: configurable report language (default zh-CN)

Add providers.yaml `language` / Action input `language`; prompts and
published summary/inline honor it. Soften fail-closed messaging when
models 429; prefer cohere/nvidia in the first-3 parallel set for dogfood.

scripts/resolve-providers.py

@@ -295,10 +295,16 @@ def main() -> None:
     )
     os.chmod(secrets_path, 0o600)
 
+    # Action input GATEKEEP_LANGUAGE overrides YAML; empty → YAML; missing → en
+    override_lang = (os.environ.get("GATEKEEP_LANGUAGE") or "").strip()
+    cfg_lang = str(cfg.get("language") or cfg.get("report_language") or "").strip()
+    language = override_lang or cfg_lang or "en"
+
     plan = {
         "agreement_min": int(cfg.get("agreement_min", 2)),
         "temperature": cfg.get("temperature", 0),
         "max_tokens": int(cfg.get("max_tokens", 4096)),
+        "language": language,
         "models": plan_models,
         "secrets_file": str(secrets_path),
         "providers_used": sorted(provider_meta.keys()),
@@ -311,6 +317,7 @@ def main() -> None:
                 "models": [f"{m['provider']}:{m['model']}" for m in plan_models],
                 "keys": {m["provider"]: m["key_name"] for m in plan_models},
                 "agreement_min": plan["agreement_min"],
+                "language": plan["language"],
             }
         ),
         flush=True,