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-test-scope.cjs

@@ -4,6 +4,8 @@
 const { execFileSync } = require('child_process');
 const { existsSync, readdirSync, appendFileSync } = require('fs');
 
+const { ExitError, runMain } = require('./lib/cli-exit.cjs');
+
 const RULES = [
   {
     name: 'workflow automation',
@@ -197,7 +199,7 @@ function parseArgs(argv) {
       if (!out.files) throw new Error('--files requires a value');
     } else if (arg === '--help' || arg === '-h') {
       console.log(usage());
-      process.exit(0);
+      throw new ExitError(0);
     } else {
       throw new Error(`unknown argument: ${arg}`);
     }
@@ -317,10 +319,11 @@ function main() {
     writeOutputs(result);
     console.log(JSON.stringify(result, null, 2));
   } catch (error) {
+    if (error instanceof ExitError) throw error;
     console.error(`ci-test-scope: ${error.message}`);
     console.error(usage());
-    process.exit(2);
+    throw new ExitError(2);
   }
 }
 
-main();
+runMain(main);