fix(ai): add fallback default model when Ollama model is empty

When model is not explicitly set in config or request, fall back to
llama3 to prevent 'model is required' errors from Ollama.

internal/ai/providers/ollama.go

@@ -98,6 +98,10 @@ func (c *OllamaClient) Chat(ctx context.Context, req ChatRequest) (*ChatResponse
 	if model == "" {
 		model = c.model
 	}
+	// Ultimate fallback - if no model configured anywhere, use llama3
+	if model == "" {
+		model = "llama3"
+	}
 
 	ollamaReq := ollamaRequest{
 		Model:    model,