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/changeset/new.cjs

@@ -13,6 +13,7 @@
 
 const fs = require('node:fs');
 const path = require('node:path');
+const { ExitError, runMain } = require('../lib/cli-exit.cjs');
 
 // Small word lists — keep the function simple and dependency-free.
 // Together this gives ~40 * 40 * 40 = 64,000 distinct names. The lint
@@ -121,17 +122,16 @@ function main() {
   if (!parsed.ok) {
     process.stderr.write(`${parsed.error}\n`);
     process.stderr.write('usage: changeset/new.cjs --type <Fixed|Added|...> --pr NNNN --body "..."\n');
-    process.exit(2);
+    throw new ExitError(2);
   }
   const { opts } = parsed;
   if (!opts.type || !opts.pr || !opts.body) {
-    process.stderr.write('usage: changeset/new.cjs --type <Fixed|Added|...> --pr NNNN --body "..."\n');
-    process.exit(2);
+    throw new ExitError(2, 'usage: changeset/new.cjs --type <Fixed|Added|...> --pr NNNN --body "..."');
   }
   const file = scaffoldFragment(opts);
   process.stdout.write(`${path.relative(process.cwd(), file)}\n`);
 }
 
-if (require.main === module) main();
+if (require.main === module) runMain(main);
 
 module.exports = { generateFragmentName, scaffoldFragment, parseArgs, ALLOWED_TYPES };