Fetch in the background and follow changes in the repositories

- On startup, after the local state is shown, fetch the superproject and
  every initialized submodule (all remotes, prune, at most 8 at once).
  Fetches never prompt; failures show git's message. --no-fetch skips it,
  f fetches again.
- The header shows "fetching n/m", then the fetch age and failures; the
  detail pane shows each submodule's last fetch and fetch errors.
- The git provider polls each repository's metadata (refs, HEAD, index,
  .gitmodules) every 2 s and reports changes. The app reloads only what
  changed, keeping old values, expansion and cursor; loads are
  single-flight with coalesced reruns. Refresh is driven by the
  repositories, so a fetch in another shell updates the view like ours.
- r reloads everything (worktree edits are not watched).
- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/submarine/main.go

@@ -19,8 +19,9 @@ import (
 
 func main() {
 	useMock := flag.Bool("mock", false, "use built-in mock data instead of a real repository")
+	noFetch := flag.Bool("no-fetch", false, "don't fetch remotes in the background on startup")
 	flag.Usage = func() {
-		fmt.Fprintln(flag.CommandLine.Output(), "usage: submarine [--mock] [dir]")
+		fmt.Fprintln(flag.CommandLine.Output(), "usage: submarine [--mock] [--no-fetch] [dir]")
 		flag.PrintDefaults()
 	}
 	flag.Parse()
@@ -55,7 +56,7 @@ func main() {
 	zone.NewGlobal()
 	defer zone.Close()
 
-	p := tea.NewProgram(tui.New(ctx, provider), tea.WithAltScreen(), tea.WithMouseCellMotion())
+	p := tea.NewProgram(tui.New(ctx, provider, tui.Options{Fetch: !*noFetch}), tea.WithAltScreen(), tea.WithMouseCellMotion())
 	if _, err := p.Run(); err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)