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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

@@ -47,6 +47,28 @@ func (m *Model) SetRoots(roots []*submodule.Submodule) {
 	}
 }
 
+// ReplaceRoots swaps in a reloaded tree. Unlike SetRoots it keeps the
+// expansion state and the cursor (by path); if the selected submodule is
+// gone, the cursor moves to the first one.
+func (m *Model) ReplaceRoots(roots []*submodule.Submodule) {
+	var path string
+	if m.cursor != nil {
+		path = m.cursor.Path
+	}
+	m.roots = roots
+	m.cursor = nil
+	for _, r := range m.rows() {
+		if r.kind == rowNode && r.node.Path == path {
+			m.cursor = r.node
+			break
+		}
+	}
+	if m.cursor == nil && len(roots) > 0 {
+		m.cursor = m.rows()[m.firstNodeRow()].node
+	}
+	m.clampOffset()
+}
+
 func (m *Model) SetSize(w, h int) {
 	m.width, m.height = w, h
 	m.clampOffset()