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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

@@ -8,3 +8,27 @@ type Provider interface {
 	Tree(ctx context.Context) (*Repo, error)
 	Status(ctx context.Context, path string) (*Status, error)
 }
+
+// Fetcher is implemented by providers that can download remote refs. Path
+// "" is the superproject. Fetch must be safe to call concurrently and must
+// never prompt the user.
+type Fetcher interface {
+	// Fetchable reports whether path is a repository that can be fetched
+	// (e.g. not an uninitialized submodule).
+	Fetchable(path string) bool
+	Fetch(ctx context.Context, path string) error
+}
+
+// Watcher is implemented by providers that notice changes made outside the
+// application, e.g. a fetch or commit in another shell. The channel is
+// closed when ctx is done.
+type Watcher interface {
+	Watch(ctx context.Context) <-chan Change
+}
+
+// Change reports that the git metadata of a repository changed.
+type Change struct {
+	Path      string // "" is the superproject
+	Local     bool   // HEAD, index or local branches changed, not only remote refs
+	Structure bool   // .gitmodules changed: submodules may have been added or removed
+}