feat(04-01): create AI client abstraction layer

- Add AIClient class wrapping AsyncOpenAI for model routing
- Support Requesty and OpenRouter as backend routers
- Add MODEL_MAP with claude, gpt, gemini short names
- Add init_ai_client/get_ai_client module functions
- Include HTTP-Referer header support for OpenRouter

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/moai/core/ai_client.py

@@ -0,0 +1,125 @@
+"""AI client abstraction for model routing.
+
+Provides AIClient class that wraps the OpenAI SDK to communicate with
+AI model routers (Requesty or OpenRouter). Both routers are OpenAI-compatible.
+"""
+
+from openai import AsyncOpenAI
+
+from moai.bot.config import BotConfig
+
+# Router base URLs
+ROUTER_URLS = {
+    "requesty": "https://router.requesty.ai/v1",
+    "openrouter": "https://openrouter.ai/api/v1",
+}
+
+# Short model names to full model identifiers
+MODEL_MAP = {
+    "claude": "anthropic/claude-sonnet-4-20250514",
+    "gpt": "openai/gpt-4o",
+    "gemini": "google/gemini-2.0-flash",
+}
+
+
+class AIClient:
+    """AI client wrapping OpenAI SDK for model routing.
+
+    Supports Requesty and OpenRouter as backend routers. Both use
+    OpenAI-compatible APIs with different base URLs and headers.
+
+    Attributes:
+        router: The router service name ("requesty" or "openrouter").
+        referer: HTTP-Referer header for OpenRouter (optional).
+    """
+
+    def __init__(self, router: str, api_key: str, referer: str | None = None) -> None:
+        """Initialize AI client.
+
+        Args:
+            router: Router service name ("requesty" or "openrouter").
+            api_key: API key for the router service.
+            referer: HTTP-Referer header for OpenRouter (optional).
+
+        Raises:
+            ValueError: If router is not supported.
+        """
+        if router not in ROUTER_URLS:
+            raise ValueError(f"Unsupported router: {router}. Use: {list(ROUTER_URLS.keys())}")
+
+        self.router = router
+        self.referer = referer
+
+        base_url = ROUTER_URLS[router]
+        self._client = AsyncOpenAI(base_url=base_url, api_key=api_key)
+
+    async def complete(
+        self,
+        model: str,
+        messages: list[dict],
+        system_prompt: str | None = None,
+    ) -> str:
+        """Get a completion from the AI model.
+
+        Args:
+            model: Model short name (e.g., "claude") or full identifier.
+            messages: List of message dicts with "role" and "content".
+            system_prompt: Optional system prompt to prepend.
+
+        Returns:
+            The model's response content as a string.
+        """
+        # Resolve short model names
+        resolved_model = MODEL_MAP.get(model, model)
+
+        # Build message list
+        full_messages = []
+        if system_prompt:
+            full_messages.append({"role": "system", "content": system_prompt})
+        full_messages.extend(messages)
+
+        # Build extra headers for OpenRouter
+        extra_headers = {}
+        if self.router == "openrouter" and self.referer:
+            extra_headers["HTTP-Referer"] = self.referer
+
+        # Make the API call
+        response = await self._client.chat.completions.create(
+            model=resolved_model,
+            messages=full_messages,
+            extra_headers=extra_headers if extra_headers else None,
+        )
+
+        return response.choices[0].message.content or ""
+
+
+# Module-level singleton
+_client: AIClient | None = None
+
+
+def init_ai_client(config: BotConfig) -> AIClient:
+    """Initialize the global AI client from config.
+
+    Args:
+        config: BotConfig instance with AI settings.
+
+    Returns:
+        The initialized AIClient instance.
+    """
+    global _client
+    _client = AIClient(config.ai_router, config.ai_api_key, config.ai_referer)
+    return _client
+
+
+def get_ai_client() -> AIClient:
+    """Get the global AI client instance.
+
+    Returns:
+        The initialized AIClient instance.
+
+    Raises:
+        RuntimeError: If AI client has not been initialized.
+    """
+    if _client is None:
+        raise RuntimeError("AI client not initialized. Call init_ai_client() first.")
+    return _client