feat: add integration tests and GitHub Actions CI

Add integration test files for all providers (apt, dpkg, pacman, apk,
dnf, rpm, flatpak, snap, pkg, detect) behind the 'integration' build
tag. Tests exercise real package operations: update, search, info,
install, verify, list, remove, and capability interfaces.

Add GitHub Actions workflow running unit tests on ubuntu-latest and
integration tests on Debian, Ubuntu, Fedora, Alpine, Arch Linux, and
Ubuntu+Flatpak containers/runners.

detect/detect_integration_test.go

@@ -0,0 +1,29 @@
+//go:build integration
+
+package detect_test
+
+import (
+	"testing"
+
+	"github.com/gogrlx/snack"
+	"github.com/gogrlx/snack/detect"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestIntegration_Detect(t *testing.T) {
+	mgr, err := detect.Default()
+	require.NoError(t, err)
+	require.NotNil(t, mgr)
+	t.Logf("Detected: %s", mgr.Name())
+
+	all := detect.All()
+	require.NotEmpty(t, all)
+	for _, m := range all {
+		t.Logf("Available: %s", m.Name())
+	}
+
+	caps := snack.GetCapabilities(mgr)
+	t.Logf("Capabilities: %+v", caps)
+	assert.NotEmpty(t, mgr.Name())
+}