fix(multiuser): close cross-user web-layer scoping holes found in review

The opt-in multi-user feature's only enforcement is web-layer scoping
(all sessions share one OS account). An adversarial review found 8 critical
+ 7 high cross-user holes that defeated it, plus mediums; all fixed here.
Single-user (flag-off) behavior stays byte-identical apart from documented
consistency deltas.

Ownership / confinement:
- DELETE /api/sessions (bulk) + /:id now owner-scope / findSessionOrFail
- quick-start, cron (create+fire), scheduled runs confine workingDir to the
  owner's space; case link/docker-link/docker-import confine the host path
- resolveCasePath no longer resolves linked cases for non-admins; foreign
  remote/docker cases are skipped (fall through to the caller's own local case)
- history, subagents/workflows, mux-sessions, orchestrator, cron run-history,
  away-digest, and remote/docker host reads are owner- or admin-scoped

Permission policy (section 6.3):
- non-granted users are downgraded at every spawn site incl. legacy
  /api/scheduled, PlanOrchestrator one-shots, remote launch, and the cron-fire
  gemini/codex bypass switches; resolveClaudeModeForUsername now fails closed

Auth / store:
- verify-first login throttle (a correct password is never locked out),
  /ws terminal subject to the change-password lockbox, cookie fast-path
  re-validates identity live, role/grant changes revoke sessions, admin delete
  runs the last-admin guard before any teardown
- users.json: distinguish missing (ENOENT) from corrupt/unreadable so a bad
  read can't overwrite all accounts; unique per-process temp write path

Event streams:
- debounced session:updated + batched task:updated, clipboard, and push
  notifications route by owner (fail closed); getLightState hides machine-wide
  globalStats from non-admins

Tests: two suites updated to assert the fixed (secure) behavior. tsc, eslint,
and test:ci all green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/route-helpers.ts

@@ -22,7 +22,7 @@ import type { AuthSessionRecord } from './ports/auth-port.js';
 import type { StaleExpirationMap } from '../utils/index.js';
 import { dataPath } from '../config/instance.js';
 import { isMultiUserMode, maxSessionsPerUser, userCasesDir } from '../config/multiuser.js';
-import { SYNTHETIC_ADMIN } from '../user-store.js';
+import { SYNTHETIC_ADMIN, findUser } from '../user-store.js';
 
 // Shared path constants used across route modules. CASES_DIR (project folders)
 // stays shared across instances; SETTINGS_PATH is per-instance runtime state.
@@ -166,6 +166,22 @@ export function isWorkingDirAllowed(user: AuthUser, workingDir: string): boolean
   return rel !== '' && !rel.startsWith('..') && !isAbsolute(rel);
 }
 
+/**
+ * Username-keyed variant of `isWorkingDirAllowed` for spawn sites that only carry
+ * an owner username (cron fire-time, scheduled-run loop) rather than a live request.
+ * Resolves the owner's role from the store; a missing/deleted user is treated as a
+ * non-privileged regular user (fails closed to their deterministic case space).
+ * No-op (true) in single-user mode or for an unset owner.
+ */
+export async function isWorkingDirAllowedForUsername(
+  username: string | undefined,
+  workingDir: string
+): Promise<boolean> {
+  if (!isMultiUserMode() || !username) return true;
+  const user = await findUser(username);
+  return isWorkingDirAllowed({ username, role: user?.role ?? 'user' }, workingDir);
+}
+
 /** Whether the caller is an admin (or single-user mode, where the sole user is admin). */
 export function isAdmin(req: FastifyRequest): boolean {
   return !isMultiUserMode() || getAuthUser(req).role === 'admin';