重构会话管理功能，由后端 opencode 发放 sessionId，后端做 scope

src/utils/fileStore.ts

@@ -149,6 +149,12 @@ export const toProjectKey = (projectId?: string) => toScopedKey("project", proje
 export const toStableId = (...parts: string[]) =>
   createHash("sha256").update(parts.join("|")).digest("hex").slice(0, 24);
 
+export const toConversationScopeKey = (
+  actorKey: string,
+  projectKey: string,
+  sessionId: string,
+) => `conversation-${toStableId(actorKey, projectKey, sessionId)}`;
+
 export const slugify = (value: string) =>
   value
     .toLowerCase()