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>

tests/auth/safe_compare.test.js

@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest';
+import { timingSafeStrEqual } from '../../lib/auth/safe_compare.js';
+
+// The owner bearer token is compared on every request. A naive === leaks
+// length+prefix via timing; a naive crypto.timingSafeEqual THROWS when the two
+// buffers differ in length. timingSafeStrEqual must be constant-time for equal
+// lengths and return false (never throw) for mismatched lengths or bad input.
+
+describe('timingSafeStrEqual', () => {
+  it('returns true for identical strings', () => {
+    expect(timingSafeStrEqual('vk_secrettoken', 'vk_secrettoken')).toBe(true);
+  });
+
+  it('returns false for same-length but different strings', () => {
+    expect(timingSafeStrEqual('aaaaaa', 'aaaaab')).toBe(false);
+  });
+
+  it('returns false (no throw) for different-length strings', () => {
+    expect(timingSafeStrEqual('short', 'a-much-longer-token')).toBe(false);
+  });
+
+  it('returns false for null / undefined / empty input', () => {
+    expect(timingSafeStrEqual(undefined, 'x')).toBe(false);
+    expect(timingSafeStrEqual('x', undefined)).toBe(false);
+    expect(timingSafeStrEqual('', '')).toBe(false);
+    expect(timingSafeStrEqual(null, null)).toBe(false);
+  });
+});