Load per-repo config from bare repos via git show

Load per-repo config from bare repos via git show

Repo::config() reads HEAD:.quire/config.fnl from bare repos, returning
a default RepoConfig when HEAD is missing (fresh repo), the file is
absent, or the :mirror key is omitted. Malformed Fennel surfaces as a
miette error with source labels pointing at the right line.

This is the foundation for per-repo mirror URLs — the config lives in
the repo's content, not in server-side operator state.

Assisted-by: GLM-5.1 via pi