fix(monopam): omit branch fragment for origin URLs, push opam-repo always
- pkg.ml: when using origin-derived HTTPS URL for opam files, omit the
branch fragment so git uses the repo default. Entry branch is only
used when falling back to the entry source URL (no origin configured).
- push.ml: always push workspace repos (opam-repo) when upstream=true,
even when all package repos are already in sync.