fix: use dot-sourcing instead of child PowerShell process

powershell -File runs script in child process — PATH changes don't
persist. Changed to Set-ExecutionPolicy + dot-sourcing (. script.ps1)
which runs in current session. Also added PATH restart hint to Codex
install/update scripts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

codex/ucodex_install.ps1

@@ -234,3 +234,6 @@ try {
 }
 
 Write-Host ""
+Write-Host "  NOTE: If 'codex' is not recognized, restart PowerShell or run:" -ForegroundColor Yellow
+Write-Host '  $env:Path = [System.Environment]::GetEnvironmentVariable("Path","Machine") + ";" + [System.Environment]::GetEnvironmentVariable("Path","User")' -ForegroundColor Yellow
+Write-Host ""