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/routers/settings.py

@@ -284,7 +284,7 @@ async def save_notifications(
 
     # Top-level notification preference
     notification_channel = notification_channel.strip()
-    if notification_channel in ("email", "nextcloud", "google_chat"):
+    if notification_channel in ("web_push", "email", "nextcloud", "google_chat"):
         channels["notification_channel"] = notification_channel
     else:
         channels.pop("notification_channel", None)