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- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).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

@@ -4,9 +4,9 @@ import (
 	"bytes"
 	"context"
 	"errors"
-	"fmt"
 	"os"
 	"os/exec"
+	"slices"
 	"strings"
 )
 
@@ -18,22 +18,40 @@ var gitEnv = append(os.Environ(),
 	"LC_ALL=C",
 )
 
+// gitError is a failed git command; Stderr is git's message.
+type gitError struct {
+	Args   []string
+	Stderr string
+	Err    error
+}
+
+func (e *gitError) Error() string {
+	msg := e.Stderr
+	if msg == "" {
+		msg = e.Err.Error()
+	}
+	return "git " + strings.Join(e.Args, " ") + ": " + msg
+}
+
+func (e *gitError) Unwrap() error { return e.Err }
+
 // run executes git in dir and returns stdout with the trailing newline
 // removed. The error includes git's stderr.
 func run(ctx context.Context, dir string, args ...string) (string, error) {
+	return runEnv(ctx, dir, nil, args...)
+}
+
+// runEnv is run with additional environment variables.
+func runEnv(ctx context.Context, dir string, env []string, args ...string) (string, error) {
 	cmd := exec.CommandContext(ctx, "git", append([]string{"-C", dir}, args...)...)
-	cmd.Env = gitEnv
+	cmd.Env = slices.Concat(gitEnv, env) // a fresh slice: run is called concurrently
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout, cmd.Stderr = &stdout, &stderr
 	if err := cmd.Run(); err != nil {
 		if ctx.Err() != nil {
 			return "", ctx.Err()
 		}
-		msg := strings.TrimSpace(stderr.String())
-		if msg == "" {
-			msg = err.Error()
-		}
-		return "", fmt.Errorf("git %s: %s", strings.Join(args, " "), msg)
+		return "", &gitError{Args: args, Stderr: strings.TrimSpace(stderr.String()), Err: err}
 	}
 	return strings.TrimRight(stdout.String(), "\n"), nil
 }