fix(flatpak): restore VersionQuerier functions lost in refactor

The capabilities_linux.go deletion moved VersionQuerier methods
(latestVersion, listUpgrades, upgradeAvailable, versionCmp) to
capabilities.go as receiver methods, but their platform implementations
were not added to flatpak_linux.go, causing build failure on Linux.

Also removes accidentally committed .DS_Store, .crush/, and AGENTS.md.

flatpak/flatpak_linux.go

@@ -198,6 +198,49 @@ func removeRepo(ctx context.Context, id string) error {
 	return err
 }
 
+func latestVersion(ctx context.Context, pkg string) (string, error) {
+	out, err := run(ctx, []string{"remote-info", "flathub", pkg})
+	if err != nil {
+		if strings.Contains(err.Error(), "exit status 1") {
+			return "", fmt.Errorf("flatpak latestVersion %s: %w", pkg, snack.ErrNotFound)
+		}
+		return "", fmt.Errorf("flatpak latestVersion: %w", err)
+	}
+	p := parseInfo(out)
+	if p == nil || p.Version == "" {
+		return "", fmt.Errorf("flatpak latestVersion %s: %w", pkg, snack.ErrNotFound)
+	}
+	return p.Version, nil
+}
+
+func listUpgrades(ctx context.Context) ([]snack.Package, error) {
+	out, err := run(ctx, []string{"remote-ls", "--updates", "--columns=name,application,version,origin"})
+	if err != nil {
+		if strings.Contains(err.Error(), "No updates") {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("flatpak listUpgrades: %w", err)
+	}
+	return parseList(out), nil
+}
+
+func upgradeAvailable(ctx context.Context, pkg string) (bool, error) {
+	upgrades, err := listUpgrades(ctx)
+	if err != nil {
+		return false, err
+	}
+	for _, u := range upgrades {
+		if u.Name == pkg || u.Description == pkg {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+func versionCmp(_ context.Context, ver1, ver2 string) (int, error) {
+	return semverCmp(ver1, ver2), nil
+}
+
 func upgradePackages(ctx context.Context, pkgs []snack.Target, opts ...snack.Option) (snack.InstallResult, error) {
 	o := snack.ApplyOptions(opts...)
 	var toUpgrade []snack.Target