Add read-only git provider

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>

internal/tui/theme/issues.go

@@ -55,8 +55,11 @@ func (t Theme) Delta(p submodule.PinInfo, base lipgloss.Style) string {
 	return s
 }
 
-// Ago formats a time relative to now, e.g. "5m ago".
+// Ago formats a time relative to now, e.g. "5m ago"; the zero time is "never".
 func Ago(t time.Time) string {
+	if t.IsZero() {
+		return "never"
+	}
 	d := time.Since(t)
 	switch {
 	case d < time.Minute: