feat: remove legacy alert rules, superseded by automations

Alert rules already auto-migrate into automations on boot and the old
engine stands down after. Deletes the rule/firing tables, routes, UI,
and AI tools; keeps notification channels since automations still use
them. Also fixes automations' notify step, which still called the
deleted repository.

src/types/ui-preferences.ts

@@ -157,7 +157,6 @@ const ALL_HIDEABLE_RAIL_VIEWS = [
   "split-screen",
   "connections",
   "session-logs",
-  "alerts",
   "fleets",
   "workspaces",
   "network_graph",