Function build_repo_filter 

pub fn build_repo_filter(
    repo_filter: &str,
    skip_filter: &str,
) -> Option<RepoFilter>

Builds a repository filter closure from --repo/--skip selectors.

Port of Go’s cmd/update.buildRepoFilter. Returns None when neither filter is set (meaning “include everything”), matching Go’s nil RepoFilterFunc return. This is pure argument-processing logic with no dependency on config loading, cluster drivers, or deployment state, so it’s implemented (and tested) directly rather than left as a red-phase stub.