feat(cli): add PowerShell completions

[Shreyasshukla957]

Summary

Adds native PowerShell completion generation for the v2 \�xecutor\ CLI. The registered completer derives subcommands, flags, and flag choices from contextual CLI help so it stays aligned with the installed command tree.

Linked issue

Fixes #2177

Verification

• \�un run format:check\ (repository baseline fails on Windows because of line-ending checks across existing files)
• \�un run lint`n- [x] \�un run typecheck`n- [ ] \�un run test\ (root script uses POSIX parameter expansion that PowerShell does not expand; the equivalent Turbo run reaches five existing Windows-only CLI failures)
• \�un run --cwd apps/cli test -- powershell-completions.test.ts`n- [x] Generated script parsed and registered in \pwsh\ using \Register-ArgumentCompleter.
• e2e: not applicable. This is generated CLI shell integration; interactive completion needs validation from an installed CLI session.

Checklist

• This change needs no changeset.
• Added tests for the new behavior.
• No secrets, credentials, or private data in the diff.