test(scripts): stop file-size fixtures leaking git repos and fsmonitor daemons (#8727)

## Summary

The file-size ratchet tests (`scripts/check-file-sizes-core.test.mjs`,
run by `just file-size-check`) created fixture git repos under the OS
tmpdir and never deleted them. When `core.fsmonitor=true` is set
globally, every git call in a fixture also started a `git
fsmonitor--daemon` that never exited. So every run left more temp repos
and more daemons behind.

- The test's `git()` helper now passes `-c core.fsmonitor=false`, next
to the existing `core.hooksPath` override.
- Fixture repos are now created through a new `createFixtureRepo()`
helper, which records each path. An `after()` hook deletes them all with
`rmSync(..., { recursive: true, force: true })`.

The other scripts that create temp git repos
(`scripts/ci-selection.test.mjs`, `scripts/flake-check.test.mjs`,
`scripts/test-mobile-release-contract.sh`,
`scripts/test-sprig-image.sh`) already clean up their temp directories
and left no daemons running, so they are unchanged.

## Testing

- `node --test scripts/check-file-sizes-core.test.mjs`: no `file-size-*`
directories are left in the tmpdir afterwards, and no new
`fsmonitor--daemon` processes are running.
- The pre-push `file-size-check` lane passes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Matt Toohey <contact@matttoohey.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

scripts/check-file-sizes-core.test.mjs

@@ -5,12 +5,13 @@ import {
   mkdirSync,
   mkdtempSync,
   realpathSync,
+  rmSync,
   symlinkSync,
   writeFileSync,
 } from "node:fs";
 import { tmpdir } from "node:os";
 import path from "node:path";
-import test from "node:test";
+import test, { after } from "node:test";
 import { policy as desktopPolicy } from "../desktop/scripts/check-file-sizes.mjs";
 import { policy as mobilePolicy } from "../mobile/scripts/check-file-sizes.mjs";
 import { policy as webPolicy } from "../web/scripts/check-file-sizes.mjs";
@@ -24,18 +25,35 @@ import {
 
 const repoRoot = path.resolve(import.meta.dirname, "..");
 
+const fixtureRepos = [];
+after(() => {
+  for (const repo of fixtureRepos)
+    rmSync(repo, { recursive: true, force: true });
+});
+
+function createFixtureRepo(prefix) {
+  const repo = realpathSync(mkdtempSync(path.join(tmpdir(), prefix)));
+  fixtureRepos.push(repo);
+  return repo;
+}
+
 function git(repo, ...args) {
   // These fixture repositories inherit both hook configuration and Git's
   // repository-local environment when this test runs from pre-push. Isolate
   // them completely so fixture commits cannot recurse into the real checkout.
+  // A global core.fsmonitor would also leave a daemon running per fixture.
   const env = Object.fromEntries(
     Object.entries(process.env).filter(([key]) => !key.startsWith("GIT_")),
   );
-  return execFileSync("git", ["-c", "core.hooksPath=/dev/null", ...args], {
-    cwd: repo,
-    encoding: "utf8",
-    env,
-  }).trim();
+  return execFileSync(
+    "git",
+    ["-c", "core.hooksPath=/dev/null", "-c", "core.fsmonitor=false", ...args],
+    {
+      cwd: repo,
+      encoding: "utf8",
+      env,
+    },
+  ).trim();
 }
 
 function createEntrypointFixture({
@@ -44,9 +62,7 @@ function createEntrypointFixture({
   lineDelta = 1,
   symlinkEntrypoint = false,
 }) {
-  const repo = realpathSync(
-    mkdtempSync(path.join(tmpdir(), `file-size-${surface}-`)),
-  );
+  const repo = createFixtureRepo(`file-size-${surface}-`);
   const scriptsDir = path.join(repo, "scripts");
   const surfaceScriptsDir = path.join(repo, surface, "scripts");
   mkdirSync(scriptsDir, { recursive: true });
@@ -95,7 +111,7 @@ function createEntrypointFixture({
 }
 
 test("local base resolution uses the branch merge-base and fails without origin/main", () => {
-  const repo = mkdtempSync(path.join(tmpdir(), "file-size-base-"));
+  const repo = createFixtureRepo("file-size-base-");
   git(repo, "init", "-b", "main");
   git(repo, "config", "user.name", "Test");
   git(repo, "config", "user.email", "test@example.com");