fix(security): IPv6 ULA blocking, cookie redaction, per-tab cancel, targeted token (#664)

Community PR #664 by @mr-k-man (security audit round 1, new parts only).

- IPv6 ULA prefix blocking (fc00::/7) in url-validation.ts with false-positive
  guard for hostnames like fd.example.com
- Cookie value redaction for tokens, API keys, JWTs in browse cookies command
- Per-tab cancel files in killAgent() replacing broken global kill-signal
- design/serve.ts: realpathSync upgrade prevents symlink bypass in /api/reload
- extension: targeted getToken handler replaces token-in-health-broadcast
- Supabase migration 003: column-level GRANT restricts anon UPDATE scope
- Telemetry sync: upsert error logging
- 10 new tests for IPv6, cookie redaction, DNS rebinding, path traversal

Co-Authored-By: mr-k-man <mr-k-man@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extension/sidepanel.js

@@ -1570,7 +1570,10 @@ chrome.runtime.onMessage.addListener((msg) => {
   if (msg.type === 'health') {
     if (msg.data) {
       const url = `http://127.0.0.1:${msg.data.port || 34567}`;
-      updateConnection(url, msg.data.token);
+      // Request token via targeted sendResponse (not broadcast) to limit exposure
+      chrome.runtime.sendMessage({ type: 'getToken' }, (resp) => {
+        updateConnection(url, resp?.token || null);
+      });
       applyChatEnabled(!!msg.data.chatEnabled);
     } else {
       updateConnection(null);