refactor(scripts): replace process.exit() with ExitError + runMain handler (#739) (#740)

Part 1 of 2 of the n/no-process-exit cleanup (umbrella #738): convert every
process.exit() call in standalone scripts/** CLIs to the rule-compliant pattern.

- New shared helper scripts/lib/cli-exit.cjs: ExitError(code,message) + runMain()
  which translates a thrown ExitError / returned number into process.exitCode
  (never process.exit()), flushing output and still firing process.on('exit').
- main()-based entrypoints: throw new ExitError(code) for errors, return <code>
  for verdicts; invoked via runMain(main). Child exit codes preserved via return.
- top-level-only scripts: imperative body extracted into main() so mid-flow
  aborts (throw ExitError) actually halt; pure consts/helpers stay at module scope.
- diff-touches-shipped-paths.cjs: stdin event handling restructured to an async
  read so the whole flow runs under runMain; uncaughtException/unhandledRejection
  nets replaced by an in-band catch that preserves EXIT_ERROR=2.

Exit codes verified unchanged for every converted script (success/error/help and
the 0/1/2 semantic codes in diff-touches). Rule stays warn here; flipped to error
in part 2 (#738) once gsd-core/bin/** is also clean.

Refs #739

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/ci-guard-runner.cjs

@@ -6,11 +6,17 @@
 // Exit 0 = github-hosted runner confirmed.
 // Exit 1 = not a github-hosted runner (emits GitHub Actions error annotation).
 
+const { ExitError, runMain } = require('./lib/cli-exit.cjs');
+
 const env = process.env.RUNNER_ENVIRONMENT || '';
 
-if (env !== 'github-hosted') {
-  process.stderr.write(
-    `::error::Expected github-hosted runner. RUNNER_ENVIRONMENT=${env || 'unset'}\n`
-  );
-  process.exit(1);
+function main() {
+  if (env !== 'github-hosted') {
+    throw new ExitError(
+      1,
+      `::error::Expected github-hosted runner. RUNNER_ENVIRONMENT=${env || 'unset'}`
+    );
+  }
 }
+
+runMain(main);