feat: remove legacy alert rules, superseded by automations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/rail-items.test.ts

@@ -92,7 +92,6 @@ describe("RAIL_ITEMS", () => {
       "ai",
       "history",
       "session-logs",
-      "alerts",
     ]);
   });
 
@@ -138,7 +137,6 @@ describe("RAIL_ITEMS", () => {
       "ai",
       "history",
       "session-logs",
-      "alerts",
     ]);
   });
 });