feat(web): add subagent conversation transport

packages/client/test-runtime/src/fixtures.ts

@@ -52,6 +52,7 @@ export function conversationSnapshot(sessionId: SessionId): ConversationSnapshot
     pending: [],
     queue: [],
     running: false,
+    subagent: null,
     composerPhase: 'active',
     removed: false,
     openState: 'open',

packages/client/test-runtime/src/sessions.ts

@@ -187,6 +187,7 @@ export class TestSessions implements ISessions {
   constructor(private readonly stabilize: Stabilizer, private readonly rootCtx: Context) {
     this.list = createSnapshotStore<SessionListState>({
       ids: [], byId: {}, current: undefined, phase: 'ready',
+      subagentsByParent: {}, currentAddress: undefined,
     })
     this.channel = new SessionProvideChannel({
       rebuildBundles: () => {
@@ -395,6 +396,11 @@ export class TestSessions implements ISessions {
     this.list.update((draft) => { draft.current = id })
   }
 
+  /** Test fixtures do not synthesize catalog addresses. */
+  subagentAddress(_id: SessionId): undefined {
+    return undefined
+  }
+
   /** Clear the current selection (recorded; the production no-session flow). */
   clear(): void {
     this.calls.push({ method: 'clear', args: [] })