refactor(hooks): remove beast-mode system integration

Remove the beast-mode-system hook and all transform wiring so Copilot-specific prompt injection is fully eliminated from the runtime pipeline.

src/plugin/system-transform.ts

@@ -1,12 +1,6 @@
-import type { CreatedHooks } from "../create-hooks"
-
-export function createSystemTransformHandler(args: {
-  hooks: CreatedHooks
-}): (input: { sessionID: string }, output: { system: string[] }) => Promise<void> {
-  return async (input, output): Promise<void> => {
-    await args.hooks.beastModeSystem?.["experimental.chat.system.transform"]?.(
-      input,
-      output,
-    )
-  }
+export function createSystemTransformHandler(): (
+  input: { sessionID: string },
+  output: { system: string[] },
+) => Promise<void> {
+  return async (): Promise<void> => {}
 }