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/hooks/index.ts

@@ -49,5 +49,4 @@ export { createTasksTodowriteDisablerHook } from "./tasks-todowrite-disabler";
 export { createRuntimeFallbackHook, type RuntimeFallbackHook, type RuntimeFallbackOptions } from "./runtime-fallback";
 export { createWriteExistingFileGuardHook } from "./write-existing-file-guard";
 export { createHashlineReadEnhancerHook } from "./hashline-read-enhancer";
-export { createBeastModeSystemHook, BEAST_MODE_SYSTEM_PROMPT } from "./beast-mode-system";
 export { createJsonErrorRecoveryHook, JSON_ERROR_TOOL_EXCLUDE_LIST, JSON_ERROR_PATTERNS, JSON_ERROR_REMINDER } from "./json-error-recovery";