Add sandbox work-folder persistence and shared lifecycle

Introduce company-scoped task, agent, user and project files, recoverable trash, object-backed repository checkpoints, and a shared 180-second lifecycle for sandbox execution. Wire file browsing and save status into the owner surfaces. This is an implementation checkpoint; staging acceptance and the complete merge gate remain outstanding.

packages/shared/src/index.ts

@@ -2728,3 +2728,4 @@ export {
   rewriteUrlHostToLoopback,
 } from "./runtime-exposure/loopback-bind.js";
 export { ACCOUNT_HANDLE_MAX_LENGTH, toAccountHandle } from "./account-handle.js";
+export * from "./work-folders.js";

packages/shared/src/work-folders.ts

@@ -0,0 +1,63 @@
+/** Durable Paperclip files; CLI homes and caches are deliberately not scopes. */
+export const WORK_FOLDER_SCOPES = ["task", "agent", "user", "project"] as const;
+export type WorkFolderScope = (typeof WORK_FOLDER_SCOPES)[number];
+export const WORK_FOLDER_SYNC_INTERVAL_MS = 180_000;
+
+export interface WorkFolderOwner {
+  companyId: string;
+  scope: WorkFolderScope;
+  ownerId: string;
+}
+
+export interface WorkFile {
+  id: string;
+  path: string;
+  kind: "file" | "directory";
+  byteSize: number;
+  sha256: string | null;
+  executable: boolean;
+  contentType: string;
+  deletedAt: string | null;
+  updatedAt: string;
+}
+
+export interface WorkFolderListing {
+  id: string;
+  owner: WorkFolderOwner;
+  files: WorkFile[];
+  nextCursor: string | null;
+}
+
+export interface WorkFolderSyncStatus {
+  runId: string;
+  state: "starting" | "saved" | "saving" | "failed";
+  lastSavedAt: string | null;
+  error: string | null;
+  refreshRequested: boolean;
+  active: boolean;
+}
+
+export interface SandboxWorkFolderManifest {
+  version: 1;
+  companyId: string;
+  runId: string;
+  taskId: string | null;
+  agentId: string;
+  responsibleUserId: string | null;
+  projectId: string | null;
+  leaseId: string;
+  sandboxKey: string;
+  home: string;
+  finalCheckpointAt?: string;
+  folders: Record<WorkFolderScope, string | null>;
+  repositories: Array<{ bindingId: string; workspaceId: string; name: string; primary: boolean }>;
+}
+
+/** Reject ambiguous paths instead of normalizing traversal into a valid path. */
+export function validateWorkFilePath(value: string): string {
+  if (!value || value.length > 1024 || /[\\\x00-\x1f\x7f]/.test(value)
+    || value.split("/").some((part) => !part || part === "." || part === ".." || part === ".paperclip-runtime")) {
+    throw new Error("Invalid work file path");
+  }
+  return value;
+}