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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

@@ -14,6 +14,7 @@ import (
 	"slices"
 	"strings"
 	"sync"
+	"time"
 
 	"submarine/internal/submodule"
 )
@@ -27,8 +28,13 @@ const maxBetween = 50
 
 // Provider reads a superproject and its submodules with git.
 type Provider struct {
-	root string        // absolute superproject root
-	sem  chan struct{} // bounds concurrent Status calls
+	root     string        // absolute superproject root
+	sem      chan struct{} // bounds concurrent Status calls
+	fetchSem chan struct{} // bounds concurrent fetches
+	sshEnv   []string      // extra environment for fetches
+
+	// WatchInterval is how often Watch checks for changes.
+	WatchInterval time.Duration
 
 	mu   sync.RWMutex
 	mods map[string]info // by Submodule.Path; filled by Tree
@@ -49,8 +55,11 @@ func New(dir string) (*Provider, error) {
 		return nil, fmt.Errorf("%s is not a git repository", dir)
 	}
 	return &Provider{
-		root: root,
-		sem:  make(chan struct{}, runtime.NumCPU()),
+		root:          root,
+		sem:           make(chan struct{}, runtime.NumCPU()),
+		fetchSem:      make(chan struct{}, maxFetches),
+		sshEnv:        sshBatchEnv(root),
+		WatchInterval: 2 * time.Second,
 	}, nil
 }
 
@@ -168,6 +177,12 @@ func status(ctx context.Context, in info) (*submodule.Status, error) {
 		return st, nil
 	}
 
+	if gitDir, _, err := gitDirs(in.dir); err == nil {
+		if fi, err := os.Stat(filepath.Join(gitDir, "FETCH_HEAD")); err == nil {
+			st.FetchedAt = fi.ModTime()
+		}
+	}
+
 	// checkout and worktree
 	head, err := run(ctx, in.dir, "rev-parse", "HEAD")
 	if err != nil {