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/mutation-matrix.cjs

@@ -32,6 +32,8 @@
 const { execFileSync } = require('child_process');
 const { readFileSync } = require('fs');
 
+const { ExitError, runMain } = require('./lib/cli-exit.cjs');
+
 // ── Single source of truth: covered modules ───────────────────────────────────
 // Each entry: { cjs: '<built artifact>', tests: ['tests/...', ...] }
 // A module is "covered" iff its tests are wired into the Stryker command runner
@@ -121,7 +123,7 @@ function parseArgs(argv) {
         '  --base <ref>   Git ref to diff against (default: origin/${GITHUB_BASE_REF:-next})',
         '  --print        Human-readable output instead of JSON',
       ].join('\n'));
-      process.exit(0);
+      throw new ExitError(0);
     } else {
       throw new Error(`unknown argument: ${arg}`);
     }
@@ -211,9 +213,10 @@ function main() {
       console.log(JSON.stringify(result, null, 2));
     }
   } catch (err) {
+    if (err instanceof ExitError) throw err;
     console.error(`mutation-matrix: ${err.message}`);
-    process.exit(2);
+    throw new ExitError(2);
   }
 }
 
-main();
+runMain(main);