Rough-in support for deepseeek-style separate reasoning output

pkg/api/api.go

@@ -16,10 +16,11 @@ const (
 )
 
 type Message struct {
-	Content     string // TODO: support multi-part messages
-	Role        MessageRole
-	ToolCalls   []ToolCall
-	ToolResults []ToolResult
+	Content          string // TODO: support multi-part messages
+	ReasoningContent string
+	Role             MessageRole
+	ToolCalls        []ToolCall
+	ToolResults      []ToolResult
 }
 
 type ToolSpec struct {
@@ -49,10 +50,11 @@ type ToolResult struct {
 	Result     string `json:"result,omitempty" yaml:"result"`
 }
 
-func NewMessageWithAssistant(content string) *Message {
+func NewMessageWithAssistant(content string, reasoning string) *Message {
 	return &Message{
 		Role:    MessageRoleAssistant,
 		Content: content,
+		ReasoningContent: reasoning,
 	}
 }