feat: implement detect package and snack CLI

- detect: auto-detection with Default(), All(), ByName()
  - Platform-specific probing via build tags (linux, freebsd, openbsd)
  - Linux probe order: apt, dnf, pacman, apk, flatpak, snap
- cmd/snack: full CLI with cobra + charmbracelet/fang
  - Subcommands: install, remove, purge, upgrade, update, list, search,
    info, which, hold, unhold, clean, detect, version
  - Global flags: --manager, --sudo, --yes, --dry-run
  - Capability type-assertions for FileOwner, Holder, Cleaner
- detect: basic compilation tests

pkg/pkg_other.go

@@ -0,0 +1,51 @@
+//go:build !freebsd
+
+package pkg
+
+import (
+	"context"
+
+	"github.com/gogrlx/snack"
+)
+
+func available() bool { return false }
+
+func install(_ context.Context, _ []snack.Target, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func remove(_ context.Context, _ []snack.Target, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func purge(_ context.Context, _ []snack.Target, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func upgrade(_ context.Context, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func update(_ context.Context) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func list(_ context.Context) ([]snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func search(_ context.Context, _ string) ([]snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func info(_ context.Context, _ string) (*snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func isInstalled(_ context.Context, _ string) (bool, error) {
+	return false, snack.ErrUnsupportedPlatform
+}
+
+func version(_ context.Context, _ string) (string, error) {
+	return "", snack.ErrUnsupportedPlatform
+}