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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

@@ -0,0 +1,43 @@
+package git
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// gitDirs returns the git directory of the worktree at dir and its common
+// directory (they differ only for linked worktrees). A submodule's .git is
+// a file "gitdir: <path>".
+func gitDirs(dir string) (gitDir, commonDir string, err error) {
+	dotGit := filepath.Join(dir, ".git")
+	fi, err := os.Stat(dotGit)
+	if err != nil {
+		return "", "", err
+	}
+	gitDir = dotGit
+	if !fi.IsDir() {
+		b, err := os.ReadFile(dotGit)
+		if err != nil {
+			return "", "", err
+		}
+		p, ok := strings.CutPrefix(strings.TrimSpace(string(b)), "gitdir:")
+		if !ok {
+			return "", "", errors.New(dotGit + ": not a gitdir file")
+		}
+		gitDir = joinPath(dir, strings.TrimSpace(p))
+	}
+	commonDir = gitDir
+	if b, err := os.ReadFile(filepath.Join(gitDir, "commondir")); err == nil {
+		commonDir = joinPath(gitDir, strings.TrimSpace(string(b)))
+	}
+	return gitDir, commonDir, nil
+}
+
+func joinPath(base, p string) string {
+	if !filepath.IsAbs(p) {
+		p = filepath.Join(base, p)
+	}
+	return filepath.Clean(p)
+}