[codex] Fix release parallel rebuild exit handling

[Samuel-Mencke]

Summary

• replaces the parallel npm rebuild process launcher with a cmd.exe-backed process so completed processes expose a reliable ExitCode
• keeps stdout/stderr logging behavior unchanged

Testing

• PowerShell parser check for scripts/build-pointer-release.ps1
• git diff --check

Context

The main release run failed after a successful native package rebuild because Start-Process returned an empty ExitCode when used without -Wait in the parallel wrapper.