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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

@@ -0,0 +1,43 @@
+package tui
+
+// treeKey is the reloader key for loading the tree; submodules use their
+// path.
+const treeKey = "\x00tree"
+
+// reloader allows at most one load per key at a time. A request while a
+// load is running marks the key dirty, and the load runs once more after
+// the current one finishes, so the last result always reflects the latest
+// change and results never arrive out of order.
+type reloader struct {
+	running map[string]bool
+	dirty   map[string]bool
+}
+
+func newReloader() reloader {
+	return reloader{running: map[string]bool{}, dirty: map[string]bool{}}
+}
+
+// start reports whether a load for key should start now. If one is already
+// running, the key is marked dirty instead.
+func (r reloader) start(key string) bool {
+	if r.running[key] {
+		r.dirty[key] = true
+		return false
+	}
+	r.running[key] = true
+	return true
+}
+
+// done marks the load for key as finished. It reports whether the load must
+// run again because it was requested meanwhile; the key then stays running.
+func (r reloader) done(key string) (again bool) {
+	if r.dirty[key] {
+		delete(r.dirty, key)
+		return true
+	}
+	delete(r.running, key)
+	return false
+}
+
+// busy reports whether any load is running.
+func (r reloader) busy() bool { return len(r.running) > 0 }