fix(auth): constant-time owner-token comparison

Owner bearer token was compared with === / !==, which short-circuits on the
first differing byte and leaks token length+prefix via response timing
(security-sweep-2026-06-01.md). New timingSafeStrEqual (crypto.timingSafeEqual
with a length pre-check so it never throws on length mismatch); wired into both
owner.js and agent_auth.js.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/auth/owner.js

@@ -1,3 +1,5 @@
+import { timingSafeStrEqual } from './safe_compare.js';
+
 export function ownerOnly(req, res, next) {
   const expected = process.env.OWNER_TOKEN;
   if (!expected) {
@@ -7,7 +9,7 @@ export function ownerOnly(req, res, next) {
   }
   const auth = req.headers.authorization || '';
   const [scheme, token] = auth.split(' ');
-  if (scheme !== 'Bearer' || token !== expected) {
+  if (scheme !== 'Bearer' || !timingSafeStrEqual(token, expected)) {
     return res.status(401).json({
       error: { code: 'unauthorized', message: 'invalid token' }
     });