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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

@@ -29,6 +29,7 @@ type Submodule struct {
 	Children []*Submodule
 	Status   *Status // nil while loading
 	Err      error   // set if loading the status failed
+	FetchErr error   // set if the last fetch failed; not an issue
 }
 
 // Presence describes whether the submodule exists on disk.
@@ -177,6 +178,7 @@ type Status struct {
 	Pinned     *Commit
 	CheckedOut *Commit
 	Tip        *Commit
-	Between    []Commit // commits that make up the pin <-> tip difference
-	Changes    *Changes // nil if the worktree is clean or the details are unknown
+	Between    []Commit  // commits that make up the pin <-> tip difference
+	Changes    *Changes  // nil if the worktree is clean or the details are unknown
+	FetchedAt  time.Time // last fetch of the submodule; zero if never
 }