feat(teacher): stream Yuxi reasoning and answer previews

shared/coding-teacher.ts

@@ -94,6 +94,8 @@ export interface TeacherRequest {
   /** Original answer retained for failed or repaired parsing; never fed back as context. */
   unparsedResponse?: string;
   toolActivity?: TeacherToolActivity[];
+  /** Provider-emitted main-thread reasoning, separate from the answer/context. */
+  reasoning?: string;
 
   intent?: TeacherRequestIntent;
   sourceConversationId?: string;