Implement submodule.Provider with the git CLI so submarine reads real
repositories: `submarine [dir]`, with `--mock` for the built-in data.
- Tree reads .gitmodules recursively; Status derives presence, checkout,
worktree, staged pin, pushed state and pin -> ref resolution from local
git commands only (no network).
- Concurrent Status calls are bounded by runtime.NumCPU().
- scripts/make-fixture.sh builds a superproject covering every state; the
integration tests run against it.
- theme.Ago shows "never" for a zero time.
- Docs updated; decisions D14-D16 added.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
