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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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>

internal/tui/tree/tree_test.go

@@ -69,6 +69,23 @@ func visible(m *Model) []string {
 	return out
 }
 
+func TestReplaceRootsKeepsState(t *testing.T) {
+	m := newTree(t)
+	m.Down()  // b
+	m.Right() // expand b, cursor on c
+	m.Right() // expand c, cursor on d
+	assertCursor(t, m, "d")
+
+	m.ReplaceRoots(fixture()) // same paths, new nodes
+	assertCursor(t, m, "d")
+	if got := len(visible(m)); got != 8 {
+		t.Errorf("visible rows = %v, want b and c still expanded", visible(m))
+	}
+
+	m.ReplaceRoots(fixture()[:1]) // only "a" left
+	assertCursor(t, m, "a")
+}
+
 func TestInitiallyCollapsedWithHeaders(t *testing.T) {
 	m := newTree(t)
 	got := visible(m)