Route tests through a #testing alias, ban ../ imports, and document the alias ownership rule

src/testing.ts

@@ -1,4 +1,6 @@
-// Test support: the current user's IDs and a fresh temporary directory.
+// Test support: the current user's IDs and a fresh temporary directory, plus
+// the source helpers that the repository-level tests in tests/ need. Those
+// tests sit outside src/ and reach it only through the `#testing` alias.
 import { mkdtemp } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -21,3 +23,8 @@ export const IS_WINDOWS: boolean = process.platform === "win32";
 export function makeTempDir(): Promise<string> {
   return mkdtemp(join(tmpdir(), "bot-agents-test-"));
 }
+
+export { output } from "./command.ts";
+export { isNotFound } from "./errors.ts";
+export { isMapping } from "./json.ts";
+export type { JsonObject } from "./json.ts";