Implement post-receive hook to push main to mirror

Implement post-receive hook to push main to mirror

Post-receive reads GIT_DIR to resolve the repo, loads per-repo config
for the mirror URL and global config for the GitHub PAT, then pushes
main to the mirror. No mirror configured = no-op. Token is passed via
GIT_CONFIG env vars, never written to disk.

Also adds Quire::repo_from_path for resolving hooks that receive GIT_DIR,
and makes Repo::git public for use in command handlers.

Assisted-by: GLM-5.1 via pi