fix(update): make permission error actionable

docs/updates.md

@@ -47,6 +47,11 @@ Dry-run downloads bounded release metadata and the checksum sidecar to validate
 asset selection and checksum availability. It does not download the executable
 archive, replace the binary, or update the automatic-check timestamp.
 
+The update destination must not be writable by its group or by other users. If
+`--update` reports that the replacement directory is writable, remove those
+permissions with `chmod go-w <directory>` (use `sudo` if needed), or install
+`fetch` in a private directory such as `~/.local/bin`.
+
 ## Automatic checks
 
 Set an interval in the configuration file:

internal/update/update_unix.go

@@ -231,7 +231,8 @@ func validateReplacementDirectory(target string) error {
 		return errors.New("replacement directory is not a real directory")
 	}
 	if info.Mode().Perm()&022 != 0 {
-		return fmt.Errorf("replacement directory is writable by group or others (mode %04o)", info.Mode().Perm())
+		dir := filepath.Dir(target)
+		return fmt.Errorf("replacement directory %q is writable by group or others (mode %04o); remove group/other write permission by running `chmod go-w` on this directory (use `sudo` if needed), or install fetch in a private directory such as `~/.local/bin`", dir, info.Mode().Perm())
 	}
 	return nil
 }

internal/update/update_unix_test.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/klauspost/compress/gzip"
@@ -170,6 +171,24 @@ func TestSelfReplaceIsAtomicAndRejectsSymlink(t *testing.T) {
 	}
 }
 
+func TestValidateReplacementDirectory_ActionableError(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.Chmod(dir, 0775); err != nil {
+		t.Fatal(err)
+	}
+
+	err := validateReplacementDirectory(filepath.Join(dir, "fetch"))
+	if err == nil {
+		t.Fatal("validateReplacementDirectory accepted a group-writable directory")
+	}
+	message := err.Error()
+	for _, want := range []string{"writable by group or others", "0775", dir, "chmod go-w", "private directory"} {
+		if !strings.Contains(message, want) {
+			t.Errorf("error = %q, want it to contain %q", message, want)
+		}
+	}
+}
+
 func TestCanReplaceFile_ReadOnlyFileWritableDirectory(t *testing.T) {
 	dir := t.TempDir()
 	path := filepath.Join(dir, "fetch")