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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

@@ -0,0 +1,31 @@
+package tui
+
+import "testing"
+
+func TestReloader(t *testing.T) {
+	r := newReloader()
+	if !r.start("a") {
+		t.Fatal("first start: want true")
+	}
+	if r.start("a") || r.start("a") {
+		t.Fatal("start while running: want false")
+	}
+	if !r.start("b") {
+		t.Fatal("other key: want true")
+	}
+	if !r.done("a") {
+		t.Fatal("done after requests while running: want again")
+	}
+	if r.done("a") {
+		t.Fatal("second done: requests were coalesced into one rerun")
+	}
+	if r.done("b") {
+		t.Fatal("done without requests: want no rerun")
+	}
+	if r.busy() {
+		t.Fatal("busy after all done")
+	}
+	if !r.start("a") {
+		t.Fatal("start after done: want true")
+	}
+}