Return the default working directory (relative to repo root) for a language’s
setup commands. Languages whose manifest file lives outside the repo root
(Swift’s Package.swift, Kotlin-Android’s gradlew, Dart’s pubspec.yaml,
Zig’s build.zig) need install commands run from their package directory or
the underlying tool will not find the manifest.
Stand-alone factory matching the spec’s setup_config_for_language(lang)
signature: returns a setup config with only the workdir field populated
(per-language default). Used by tests and callers that only need the workdir
default without resolving the full command pipeline (which requires
output_dir and LangContext).