feat: add claude-code-router support for multi-model testing

- Add ROUTER=true flag to route requests through claude-code-router
- Add router service to docker-compose with profile-based activation
- Support OpenAI (gpt-4o) and Google Gemini (gemini-2.5-pro) as alternatives
- Add router-config.json with provider configuration template
- Update .env.example with provider API key options
- Document router mode limitations (cost tracking shows $0)

configs/router-config.json

@@ -0,0 +1,31 @@
+{
+  "HOST": "0.0.0.0",
+  "APIKEY": "shannon-router-key",
+  "LOG": true,
+  "LOG_LEVEL": "info",
+  "NON_INTERACTIVE_MODE": true,
+  "API_TIMEOUT_MS": 600000,
+  "Providers": [
+    {
+      "name": "openai",
+      "api_base_url": "https://api.openai.com/v1/chat/completions",
+      "api_key": "$OPENAI_API_KEY",
+      "models": ["gpt-4o", "gpt-4o-mini"],
+      "transformer": {
+        "use": [["maxtoken", { "max_tokens": 16384 }]]
+      }
+    },
+    {
+      "name": "gemini",
+      "api_base_url": "https://generativelanguage.googleapis.com/v1beta/models/",
+      "api_key": "$GEMINI_API_KEY",
+      "models": ["gemini-2.5-pro", "gemini-2.5-flash"],
+      "transformer": {
+        "use": ["gemini"]
+      }
+    }
+  ],
+  "Router": {
+    "default": "$ROUTER_DEFAULT"
+  }
+}