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/release-notes/format-github-release-notes.cjs

@@ -4,6 +4,7 @@ const path = require('path');
 const os = require('os');
 const fs = require('fs');
 const { execFileSync } = require('child_process');
+const { runMain, ExitError } = require('../lib/cli-exit.cjs');
 
 /**
  * Classify a What's-Changed bullet line into 'Feature', 'Fix', or 'Enhancement'.
@@ -155,7 +156,7 @@ function formatReleaseNotes({ generatedBody, version, prerelease, packageName })
 }
 
 // CLI entry point
-if (require.main === module) {
+function main() {
   try {
     const argv = process.argv.slice(2);
 
@@ -248,9 +249,13 @@ if (require.main === module) {
       process.stdout.write(formatted + '\n');
     }
   } catch (err) {
-    process.stderr.write((err.message || String(err)) + '\n');
-    process.exit(1);
+    if (err instanceof ExitError) throw err;
+    throw new ExitError(1, err && err.message ? err.message : String(err));
   }
 }
 
+if (require.main === module) {
+  runMain(main);
+}
+
 module.exports = { formatReleaseNotes, classifyTitle };