perf(slack): narrow runtime-setter + lazy-load 4 modules + narrow 2 SDK surfaces (#69317)

Lazy load modules showing a ~50% gateway startup performance improvement

extensions/slack/runtime-setter-api.ts

@@ -0,0 +1,3 @@
+// Narrow entry point for setSlackRuntime — avoids pulling in the full
+// runtime-api barrel (284KB, 29 chunks) during plugin register().
+export { setSlackRuntime } from "./src/runtime.js";