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/capabilities.go

@@ -0,0 +1,44 @@
+package flatpak
+
+import (
+	"context"
+
+	"github.com/gogrlx/snack"
+)
+
+// Compile-time interface checks.
+var (
+	_ snack.Cleaner     = (*Flatpak)(nil)
+	_ snack.RepoManager = (*Flatpak)(nil)
+)
+
+// Autoremove removes unused runtimes and extensions.
+func (f *Flatpak) Autoremove(ctx context.Context, opts ...snack.Option) error {
+	f.Lock()
+	defer f.Unlock()
+	return autoremove(ctx, opts...)
+}
+
+// Clean is a no-op for flatpak (no direct cache clean equivalent).
+func (f *Flatpak) Clean(ctx context.Context) error {
+	return nil
+}
+
+// ListRepos returns all configured remotes.
+func (f *Flatpak) ListRepos(ctx context.Context) ([]snack.Repository, error) {
+	return listRepos(ctx)
+}
+
+// AddRepo adds a new remote.
+func (f *Flatpak) AddRepo(ctx context.Context, repo snack.Repository) error {
+	f.Lock()
+	defer f.Unlock()
+	return addRepo(ctx, repo)
+}
+
+// RemoveRepo removes a configured remote.
+func (f *Flatpak) RemoveRepo(ctx context.Context, id string) error {
+	f.Lock()
+	defer f.Unlock()
+	return removeRepo(ctx, id)
+}