modules/git/remotes.nix declares upstream/fork/extras schema. extras is
a typed submodule list so order is preserved and future fields
(pushUrl, mirror) can extend without breaking callers. docs/remotes.md
walks the three canonical topologies (upstream-only / github-fork /
multi-remote with private host).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
