feat: proactive notifications — web_push channel + daily reminder check

Routes web_push through notification.py alongside NCT/email/Google Chat,
and fires daily reminder summaries via the scheduler.

- notification.py: _notify_web_push() + "web_push" case in notify();
  all four channels (web_push/email/nextcloud/google_chat) now routable
- scheduler.py: _run_reminder_check() daily at 09:00 — reads due reminders
  per persona via set_context(), formats up to 3 entries, calls notify()
- routers/settings.py: "web_push" added to valid notification_channel values
- static/settings.html: "Browser Push Notification" option in channel selector
- TODO__Agents.md: proactive notifications section marked complete

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/settings.html

@@ -361,6 +361,7 @@
                          color:var(--pg-text); font-size:0.95rem; outline:none;
                          transition:border-color 0.15s;">
             <option value="">None (disabled)</option>
+            <option value="web_push">Browser Push Notification</option>
             <option value="email">Email</option>
             <option value="nextcloud">Nextcloud Talk</option>
             <option value="google_chat">Google Chat</option>