feat(web): add nested subagent conversations

packages/client/ui-conversation/tests/input-bar.spec.tsx

@@ -26,7 +26,7 @@ function snapshotOf(overrides: Partial<ConversationSnapshot> = {}): Conversation
     sessionId: SID, nodes: [], partial: null, runningCalls: [], codeDispatches: new Map(),
     pending: [], queue: [], running: false, composerPhase: 'active', removed: false,
     openState: 'open', openError: null, hasMore: false, loadingOlder: false,
-    promptError: null, blank: false, lastAgentError: null,
+    promptError: null, blank: false, subagent: null, lastAgentError: null,
     ...overrides,
   }
 }
@@ -94,6 +94,7 @@ function bench(over?: BenchOptions) {
     useSession: bindSnapshotSelector(session),
     useSessions: bindSnapshotSelector(createSnapshotStore({
       ids: [], byId: {}, current: undefined, phase: 'ready',
+      subagentsByParent: {}, currentAddress: undefined,
     })),
     useWorkspaces: bindSnapshotSelector(createSnapshotStore({
       items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,