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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

docs/state-model.md

@@ -33,6 +33,11 @@ in several at once.
 | `Pinned`, `CheckedOut`, `Tip` | Commits (SHA, subject, author, time) |
 | `Between` | Commits that make up the pin ↔ tip difference |
 | `Changes` | Changed files of the worktree (nil if clean), see below |
+| `FetchedAt` | Last fetch of the submodule (`FETCH_HEAD` mtime); zero if never |
+
+`Submodule.FetchErr` is set if the application's last fetch of the
+submodule failed. Like loading errors, it is not an issue kind: a failed
+fetch says nothing about the submodule's state.
 
 `Status == nil` means "still loading". `Submodule.Err` is set if loading
 failed.