feat: add flatpak and snap package manager implementations

flatpak: implements Manager, Cleaner, and RepoManager interfaces
- Install/Remove/Purge/Upgrade with flatpak CLI
- Repository management (add/remove/list remotes)
- Autoremove unused runtimes

snap: implements Manager and VersionQuerier interfaces
- Install with --classic/--channel support via Target.FromRepo
- Remove/Purge/Upgrade via snap CLI
- Version queries with semver comparison
- Upgrade availability via snap refresh --list

Both packages follow the existing pattern:
- Exported methods on struct delegate to unexported functions
- _linux.go for real implementation, _other.go stubs
- Compile-time interface checks
- Parser tests for all output formats

flatpak/flatpak_linux.go

@@ -0,0 +1,151 @@
+//go:build linux
+
+package flatpak
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os/exec"
+	"strings"
+
+	"github.com/gogrlx/snack"
+)
+
+func available() bool {
+	_, err := exec.LookPath("flatpak")
+	return err == nil
+}
+
+func run(ctx context.Context, args []string) (string, error) {
+	c := exec.CommandContext(ctx, "flatpak", args...)
+	var stdout, stderr bytes.Buffer
+	c.Stdout = &stdout
+	c.Stderr = &stderr
+	err := c.Run()
+	if err != nil {
+		se := stderr.String()
+		if strings.Contains(se, "permission denied") || strings.Contains(se, "requires root") {
+			return "", fmt.Errorf("flatpak: %w", snack.ErrPermissionDenied)
+		}
+		return "", fmt.Errorf("flatpak: %s: %w", strings.TrimSpace(se), err)
+	}
+	return stdout.String(), nil
+}
+
+func install(ctx context.Context, pkgs []snack.Target, opts ...snack.Option) error {
+	_ = snack.ApplyOptions(opts...)
+	for _, t := range pkgs {
+		remote := t.FromRepo
+		if remote == "" {
+			remote = "flathub"
+		}
+		args := []string{"install", "-y", remote, t.Name}
+		if _, err := run(ctx, args); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func remove(ctx context.Context, pkgs []snack.Target, _ ...snack.Option) error {
+	args := append([]string{"uninstall", "-y"}, snack.TargetNames(pkgs)...)
+	_, err := run(ctx, args)
+	return err
+}
+
+func purge(ctx context.Context, pkgs []snack.Target, _ ...snack.Option) error {
+	args := append([]string{"uninstall", "-y", "--delete-data"}, snack.TargetNames(pkgs)...)
+	_, err := run(ctx, args)
+	return err
+}
+
+func upgrade(ctx context.Context, _ ...snack.Option) error {
+	_, err := run(ctx, []string{"update", "-y"})
+	return err
+}
+
+func list(ctx context.Context) ([]snack.Package, error) {
+	out, err := run(ctx, []string{"list", "--columns=name,application,version,origin"})
+	if err != nil {
+		return nil, fmt.Errorf("flatpak list: %w", err)
+	}
+	return parseList(out), nil
+}
+
+func search(ctx context.Context, query string) ([]snack.Package, error) {
+	out, err := run(ctx, []string{"search", query, "--columns=name,application,version,remotes"})
+	if err != nil {
+		if strings.Contains(err.Error(), "No matches found") {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("flatpak search: %w", err)
+	}
+	return parseSearch(out), nil
+}
+
+func info(ctx context.Context, pkg string) (*snack.Package, error) {
+	out, err := run(ctx, []string{"info", pkg})
+	if err != nil {
+		if strings.Contains(err.Error(), "exit status 1") {
+			return nil, fmt.Errorf("flatpak info %s: %w", pkg, snack.ErrNotFound)
+		}
+		return nil, fmt.Errorf("flatpak info: %w", err)
+	}
+	p := parseInfo(out)
+	if p == nil {
+		return nil, fmt.Errorf("flatpak info %s: %w", pkg, snack.ErrNotFound)
+	}
+	p.Installed = true
+	return p, nil
+}
+
+func isInstalled(ctx context.Context, pkg string) (bool, error) {
+	c := exec.CommandContext(ctx, "flatpak", "info", pkg)
+	err := c.Run()
+	if err != nil {
+		if exitErr, ok := err.(*exec.ExitError); ok && exitErr.ExitCode() == 1 {
+			return false, nil
+		}
+		return false, fmt.Errorf("flatpak isInstalled: %w", err)
+	}
+	return true, nil
+}
+
+func version(ctx context.Context, pkg string) (string, error) {
+	out, err := run(ctx, []string{"info", pkg})
+	if err != nil {
+		if strings.Contains(err.Error(), "exit status 1") {
+			return "", fmt.Errorf("flatpak version %s: %w", pkg, snack.ErrNotInstalled)
+		}
+		return "", fmt.Errorf("flatpak version: %w", err)
+	}
+	p := parseInfo(out)
+	if p == nil || p.Version == "" {
+		return "", fmt.Errorf("flatpak version %s: %w", pkg, snack.ErrNotInstalled)
+	}
+	return p.Version, nil
+}
+
+func autoremove(ctx context.Context, _ ...snack.Option) error {
+	_, err := run(ctx, []string{"uninstall", "--unused", "-y"})
+	return err
+}
+
+func listRepos(ctx context.Context) ([]snack.Repository, error) {
+	out, err := run(ctx, []string{"remotes", "--columns=name,url,options"})
+	if err != nil {
+		return nil, fmt.Errorf("flatpak listRepos: %w", err)
+	}
+	return parseRemotes(out), nil
+}
+
+func addRepo(ctx context.Context, repo snack.Repository) error {
+	_, err := run(ctx, []string{"remote-add", "--if-not-exists", repo.Name, repo.URL})
+	return err
+}
+
+func removeRepo(ctx context.Context, id string) error {
+	_, err := run(ctx, []string{"remote-delete", id})
+	return err
+}