feat: vendor permission system source

pi-permission-system/src/safe-system-paths.ts

@@ -0,0 +1,18 @@
+/**
+ * Paths that are universally safe and should never trigger external-directory checks.
+ * These are OS device files: read returns EOF or process streams, write discards or goes to process streams.
+ */
+export const SAFE_SYSTEM_PATHS: ReadonlySet<string> = new Set([
+  "/dev/null",
+  "/dev/stdin",
+  "/dev/stdout",
+  "/dev/stderr",
+]);
+
+/**
+ * Returns true if the given normalized path is a safe OS device file
+ * that should never trigger external-directory checks.
+ */
+export function isSafeSystemPath(normalizedPath: string): boolean {
+  return SAFE_SYSTEM_PATHS.has(normalizedPath);
+}