Add OpenRouter reasoning support

internal/llm/client_completions.go

@@ -444,6 +444,13 @@ func (c *Client) completionsStream(ctx context.Context, messages []Message, mode
 					ReasoningDelta: delta.ReasoningContent,
 				}
 			}
+			if delta.Reasoning != "" {
+				acc.reasoning.WriteString(delta.Reasoning)
+				events <- StreamEvent{
+					Type:           streamEventReasoningDelta,
+					ReasoningDelta: delta.Reasoning,
+				}
+			}
 			// Accumulate tool call deltas by index.
 			for _, tc := range delta.ToolCalls {
 				idx := 0