feat: Groq fallback writer 추가 — Gemini rate limit 시 자동 전환

- GroqWriter 클래스 추가 (llama-3.3-70b-versatile)
- FallbackWriter 래퍼: primary 실패/빈응답 → fallback chain 자동 시도
- engine.json에 groq 설정 + fallback_chain: ["groq"] 추가

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

config/engine.json

@@ -3,7 +3,8 @@
   "_updated": "2026-03-29",
   "writing": {
     "provider": "gemini",
-    "_comment_provider": "openclaw=ChatGPT Pro(OAuth), claude_web=Claude Max(웹쿠키), gemini_web=Gemini Pro(웹쿠키), claude=Anthropic API키, gemini=Google AI API키",
+    "fallback_chain": ["groq"],
+    "_comment_provider": "openclaw=ChatGPT Pro(OAuth), claude_web=Claude Max(웹쿠키), gemini_web=Gemini Pro(웹쿠키), claude=Anthropic API키, gemini=Google AI API키, groq=Groq API키",
     "options": {
       "openclaw": {
         "agent_name": "blog-writer",
@@ -29,6 +30,12 @@
         "model": "gemini-2.5-flash",
         "max_tokens": 4096,
         "temperature": 0.7
+      },
+      "groq": {
+        "api_key_env": "GROQ_API_KEY",
+        "model": "llama-3.3-70b-versatile",
+        "max_tokens": 4096,
+        "temperature": 0.7
       }
     }
   },