Add Phase 3: LLM integration with Ollama streaming and preflight checks

Wire the REPL to a local Ollama instance via streaming HTTP (SSE).
LLMClient handles async streaming chat, StreamHandler renders live
Markdown via Rich and accumulates tool call fragments. Startup now
runs a preflight check that verifies Ollama is reachable and the
configured model is pulled, exiting with a clear message on failure.
Also adds .gitignore and updates config to use qwen3.5.

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

app/services/llm.py

@@ -59,6 +59,48 @@ class LLMClient:
             timeout=httpx.Timeout(config.timeout, connect=10.0),
         )
 
+    async def preflight_check(self) -> None:
+        """Verify the endpoint is reachable and the configured model is available.
+
+        Raises:
+            LLMConnectionError: If the endpoint is unreachable.
+            LLMResponseError: If the model is not found or the endpoint returns an error.
+        """
+        # Check endpoint is reachable
+        try:
+            response = await self._client.get("/api/tags")
+        except (httpx.ConnectError, httpx.HTTPError, OSError) as e:
+            raise LLMConnectionError(
+                f"Cannot reach Ollama at {self._config.endpoint}. Is Ollama running?"
+            ) from e
+        except httpx.TimeoutException as e:
+            raise LLMConnectionError(
+                f"Timed out connecting to {self._config.endpoint}."
+            ) from e
+
+        if response.status_code != 200:
+            raise LLMResponseError(
+                f"Ollama returned {response.status_code} from /api/tags.",
+                status_code=response.status_code,
+            )
+
+        # Check model is available
+        try:
+            data = response.json()
+        except (ValueError, KeyError):
+            logger.warning("preflight_parse_error", msg="Could not parse /api/tags response")
+            return
+
+        available = [m.get("name", "") for m in data.get("models", [])]
+        model = self._config.model
+
+        # Match with or without tag suffix (e.g. "qwen3.5" matches "qwen3.5:latest")
+        if not any(model == name or model == name.split(":")[0] for name in available):
+            available_str = ", ".join(available) if available else "(none)"
+            raise LLMResponseError(
+                f"Model '{model}' not found. Available models: {available_str}"
+            )
+
     async def stream_chat(self, messages: list[Message]) -> AsyncIterator[dict]:
         """Stream a chat completion request, yielding parsed SSE chunks.