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/sse-stream-manager.ts

@@ -413,7 +413,11 @@ export class SseStreamManager {
       return;
     }
     for (const [, { sessionId, task }] of this.taskUpdateBatches) {
-      this.broadcast(SseEvent.TaskUpdated, { sessionId, task });
+      // Multi-user: batched task updates carry session state — route to the owner
+      // only (fail closed if unknown), matching flushSessionTerminalBatch. No-op for
+      // identity-less single-user clients (canDeliver short-circuits on no identity).
+      const owner = this.deps.resolveSessionOwner?.(sessionId);
+      this.broadcast(SseEvent.TaskUpdated, { sessionId, task }, { owner, sessionScoped: true });
     }
     this.taskUpdateBatches.clear();
   }
@@ -453,7 +457,11 @@ export class SseStreamManager {
       // Single expensive serialization per batch interval
       const state = this.deps.getSessionStateWithRespawn(sessionId);
       if (state) {
-        this.broadcast(SseEvent.SessionUpdated, state);
+        // Multi-user: the debounced session:updated blob carries name/workingDir/
+        // tokens/cost — route to the session owner only (fail closed if unknown),
+        // matching flushSessionTerminalBatch. No-op for single-user clients.
+        const owner = this.deps.resolveSessionOwner?.(sessionId);
+        this.broadcast(SseEvent.SessionUpdated, state, { owner, sessionScoped: true });
       }
     }
     this.stateUpdatePending.clear();