fix(build): add explicit exit 0 on success (PowerShell)

Without it, a stale $LASTEXITCODE left over from an earlier native
command in the same PowerShell session could cause a CI step to be
reported as failed even though the script itself completed successfully.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/build.ps1

@@ -173,3 +173,4 @@ $leftoverPlaceholders = @(Get-ChildItem $output -File -Recurse | ForEach-Object
 if ($leftoverPlaceholders.Count) { throw "Unresolved template placeholders in: $($leftoverPlaceholders -join ', ')" }
 
 Write-Output 'PASS build: codex-windows, claude-windows, codex-linux, claude-linux'
+exit 0