refactor(test): remove unused test helper files

Remove 4 unused test helper files with zero usages across the codebase:
- async-sync-test-helper.mts (96 lines, 0 usages)
- error-test-helper.mts (80 lines, 0 usages)
- flag-validator-helper.mts (124 lines, 0 usages)
- subprocess.mts (43 lines, 0 usages)

Total: 343 lines of dead code removed

Verification:
- Grep confirmed zero imports or references
- All tests pass after removal
- No broken dependencies

Author: jdalton