chore(deps): upgrade Commander to 15 (#1577)

## Description

Upgrade the CLI runtime from Commander 14 to 15. The CLI already uses ESM and requires Node 24, so no command implementation changes were needed.

Tracks `codemem-jnd6.7.2`.

## Type of Change

- [ ] 🚀 Feature (new functionality)
- [ ] 🐛 Bug fix (fixes an issue)
- [ ] 📚 Documentation (docs-only change)
- [x] 🔧 Maintenance (refactor, chore, CI, etc.)
- [ ] 🧪 Testing (test-only changes)

## Testing

- [x] Relevant checks pass locally (`pnpm run tsc`, `pnpm run lint`, `pnpm run test`)
- [ ] Added/updated tests for changes
- [x] Manually verified changes work as expected

Additional validation: CLI typecheck, 515 CLI tests, CLI production build, packed-artifact smoke, CLI help output, full `pnpm run check` (5,740 tests), and a high-severity Snyk scan with zero findings.

## Checklist

- [x] Code follows project style (`pnpm run lint` passes for touched files)
- [x] Self-review completed
- [x] Documentation updated (not needed; no user-facing behavior changed)
- [x] No new warnings introduced