Fix draft agent handoff continuity

Move optimistic user message reconciliation into the stream reducers and keep draft-created agents in a handoff lifecycle until authoritative history lands.

packages/app/src/stores/create-flow-store.ts

@@ -4,7 +4,7 @@ import type { AgentAttachment } from "@server/shared/messages";
 
 export type CreateFlowLifecycleState = "active" | "abandoned" | "sent";
 
-interface PendingCreateAttempt {
+export interface PendingCreateAttempt {
   draftId: string;
   serverId: string;
   agentId: string | null;
@@ -23,6 +23,7 @@ interface CreateFlowState {
   markLifecycle: (input: { draftId: string; lifecycle: CreateFlowLifecycleState }) => void;
   rekeyDraft: (input: { fromDraftId: string; toDraftId: string }) => void;
   clear: (input: { draftId: string }) => void;
+  clearByAgent: (input: { serverId: string; agentId: string }) => void;
   clearAll: () => void;
 }
 
@@ -89,5 +90,17 @@ export const useCreateFlowStore = create<CreateFlowState>((set) => ({
       const { [draftId]: _removed, ...rest } = state.pendingByDraftId;
       return { pendingByDraftId: rest };
     }),
+  clearByAgent: ({ serverId, agentId }) =>
+    set((state) => {
+      const next = Object.fromEntries(
+        Object.entries(state.pendingByDraftId).filter(
+          ([, pending]) => pending.serverId !== serverId || pending.agentId !== agentId,
+        ),
+      );
+      if (Object.keys(next).length === Object.keys(state.pendingByDraftId).length) {
+        return state;
+      }
+      return { pendingByDraftId: next };
+    }),
   clearAll: () => set({ pendingByDraftId: {} }),
 }));