Add teacher microphone and per-question model selection

shared/coding-teacher.ts

@@ -71,6 +71,8 @@ export interface TeacherRequest {
   projectId?: string;
   projectName?: string;
   teacherVersion?: number;
+  /** Explicit per-question model; absent follows the published teacher configuration. */
+  modelId?: string;
   runtimeThreadId?: string;
   origin?: { projectId: string; topicId: string; requestId: string };
   /** discussion-v1 is retained only to identify historical replies. */
@@ -147,7 +149,13 @@ export interface TeacherTopicList {
   items: Array<{ id: string; title: string; updatedAt: string; version: number; teacherId?: string; legacyRole?: 'friend'; legacySourceId?: string }>;
   lastSelectedTopicId: string | null;
 }
+export interface TeacherModelCatalog {
+  supported: boolean;
+  models: Array<{ id: string }>;
+  message?: string;
+}
 export interface TeacherSend {
+  modelId?: string;
   projectId?: string;
   intent?: TeacherRequestIntent;
   sourceConversationId?: string;