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/lint-docs-required.cjs

@@ -14,6 +14,7 @@
  */
 
 const { parseFragment, FRAGMENT_ERROR } = require('./changeset/parse.cjs');
+const { ExitError, runMain } = require('./lib/cli-exit.cjs');
 
 const LINT_REASON = Object.freeze({
   OK_NO_TRIGGERING_FRAGMENTS: 'ok_no_triggering_fragments',
@@ -161,8 +162,7 @@ function main() {
     );
     changedFiles = out.split('\n').filter(Boolean);
   } catch (e) {
-    process.stderr.write(`could not compute diff: ${e.message}\n`);
-    process.exit(2);
+    throw new ExitError(2, `could not compute diff: ${e.message}`);
   }
 
   const { fragments, malformed } = readFragmentsFromDisk(changedFiles, rootDir);
@@ -204,10 +204,10 @@ function main() {
       `exemption via \`<!-- docs-exempt: <reason> -->\` inside the fragment body also works.\n`,
     );
   }
-  process.exit(verdict.ok ? 0 : 1);
+  return verdict.ok ? 0 : 1;
 }
 
-if (require.main === module) main();
+if (require.main === module) runMain(main);
 
 module.exports = {
   evaluateLint,