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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

@@ -18,13 +18,13 @@ bare commit hash.
 ## Status
 
 **Prototype.** The UI is complete for browsing. It reads real repositories
-through a read-only git provider (`internal/submodule/git`):
-`submarine [dir]`. `submarine --mock` shows the built-in mock data
-(`internal/submodule/mock`). Actions (`u`, `b`, `f`, `/`, `!`) only show a
-"not implemented" message.
+through a git provider (`internal/submodule/git`): `submarine [dir]`. It
+fetches in the background on startup (`--no-fetch` to skip; `f` to fetch
+again) and follows changes made in other shells. `submarine --mock` shows
+the built-in mock data (`internal/submodule/mock`). The actions `u`, `b`,
+`/`, `!` only show a "not implemented" message.
 
 Next steps, roughly in order:
 
-1. Actions: fetch, checkout pin (`submodule update`), bump pin to tip, init.
+1. Actions: checkout pin (`submodule update`), bump pin to tip, init.
 2. Filter (`/`) and "problems only" (`!`) views.
-3. Refresh after actions and on demand.