fix: replace import.meta.url with process.cwd() in testkit.ts

import.meta.url is undefined when esbuild bundles to CJS format
(format: 'cjs' in build.ts). fileURLToPath(undefined) throws
ERR_INVALID_ARG_TYPE which crashed the production server on startup.

Fix: drop the _dirname derivation entirely and resolve TIMMY_TEST_PLAN.md
relative to process.cwd(), which is always the workspace root in both:
- dev: tsx runs from workspace root
- production: pnpm --filter ... run start runs from workspace root

Also removes the now-unused 'dirname' and 'fileURLToPath' imports.

Verified: rebuilt dist/index.cjs — 0 import.meta.url references remain.
GET /api/testkit/plan returns HTTP 200 from the production bundle.

artifacts/api-server/src/routes/testkit.ts

@@ -1,7 +1,6 @@
 import { Router, type Request, type Response } from "express";
 import { readFileSync } from "fs";
-import { resolve, dirname } from "path";
-import { fileURLToPath } from "url";
+import { resolve } from "path";
 
 const router = Router();
 
@@ -1105,8 +1104,7 @@ if [[ "\$FAIL" -gt 0 ]]; then exit 1; fi
  * Returns TIMMY_TEST_PLAN.md verbatim as text/markdown.
  * Path resolves from the project root regardless of cwd.
  */
-const _dirname = dirname(fileURLToPath(import.meta.url));
-const PLAN_PATH = resolve(_dirname, "../../../../TIMMY_TEST_PLAN.md");
+const PLAN_PATH = resolve(process.cwd(), "TIMMY_TEST_PLAN.md");
 
 router.get("/testkit/plan", (_req: Request, res: Response) => {
   let content: string;