feat: better support pnpm symlinked node modules

Latest NextJS evals with Turbopack fail with errors like:

```
[6:57 PM]> Build error occurred
Error [TurbopackInternalError]: Symlink node_modules is invalid, it points out of the filesystem root

Debug info:
- Execution of get_all_written_entrypoints_with_issues_operat
```

We can fix this by operating in temporary workspace directories _inside_ the ".web-codegen-scorer" directory.

Author: devversion

runner/configuration/constants.ts

@@ -23,6 +23,15 @@ export const DEFAULT_SUMMARY_MODEL = 'gemini-2.5-flash-lite';
 /** Name of the root folder where we store LLM-generated code for debugging */
 export const LLM_OUTPUT_DIR = join(rootDir, 'llm-output');
 
+/**
+ * Path for WCS temporary directories.
+ *
+ * We want temporary workspaces to be nested inside the root project to
+ * better support symlinked node modules. E.g. Turbopack will otherwise
+ * fail if symlinked node modules are not reachable via parent directories.
+ */
+export const WCS_BASE_TMP_DIR = join(rootDir, 'tmp-workspaces');
+
 /**
  * Number of times we'll try to ask LLM to repair a build failure,
  * providing the build output and the code that causes the problem.

runner/orchestration/file-system.ts

@@ -1,5 +1,4 @@
-import {tmpdir} from 'os';
-import {LLM_OUTPUT_DIR} from '../configuration/constants.js';
+import {LLM_OUTPUT_DIR, WCS_BASE_TMP_DIR} from '../configuration/constants.js';
 import {Environment} from '../configuration/environment.js';
 import {
   copyFolderExcept,
@@ -25,13 +24,15 @@ const PENDING_INSTALLS = new Map<string, Promise<void>>();
  * @param env Environment that is currently being run.
  * @param rootPromptDef Definition of the root prompt.
  * @param progress Logger to use to log out the current progress.
+ * @param tmpdirBasePath Base path for temporary directories (like `/tmp`).
  * @param outputDirectory Custom output directory specified by the user.
  * @returns Temporary directory in which to build and a function used to clean in up.
  */
 export async function setupProjectStructure(
   env: Environment,
   rootPromptDef: RootPromptDefinition,
   progress: ProgressLogger,
+  tmpdirBasePath: string = WCS_BASE_TMP_DIR,
   outputDirectory?: string,
 ) {
   let directory: string;
@@ -48,7 +49,8 @@ export async function setupProjectStructure(
     cleanup = () => Promise.resolve();
   } else {
     // When outputting to the temporary directory, make sure that the directory is unique.
-    directory = await mkdtemp(join(tmpdir(), `fw-${env.id}-build-${rootPromptDef.name}`));
+    await mkdir(tmpdirBasePath, {recursive: true});
+    directory = await mkdtemp(join(tmpdirBasePath, `fw-${env.id}-build-${rootPromptDef.name}`));
 
     cleanup = async () => {
       try {