fix: remove completion_args from agent conversation request

Mistral rejects completion_args when agent_id is set. The agent's
own configuration (set in Mistral UI) controls response format.

backend/internal/pkg/ai/client.go

@@ -59,9 +59,6 @@ func (c *Client) Pass1(ctx context.Context, prompt string) (PassResult, error) {
 		AgentID: c.agentSimple,
 		Inputs:  conversation.TextInputs(prompt),
 		Store:   &storeFalse,
-		CompletionArgs: &conversation.CompletionArgs{
-			ResponseFormat: &chat.ResponseFormat{Type: "text"},
-		},
 	})
 	if err != nil {
 		return PassResult{}, fmt.Errorf("pass1 conversation: %w", err)