feat: remove legacy alert rules, superseded by automations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/ui/tests/sidebar/plugin-extension-seam.test.tsx

@@ -29,10 +29,6 @@ vi.mock("react-i18next", () => ({
   useTranslation: () => ({ t: (key: string) => key }),
 }));
 
-vi.mock("@/api/alerts-api", () => ({
-  getAlertFirings: vi.fn().mockResolvedValue([]),
-}));
-
 vi.mock("@/hooks/use-ai-availability", () => ({
   useAiAvailability: () => ({ userEnabled: false }),
 }));