refactor: move internal/ to pkg/ for external importability

Moves all packages from internal/ to pkg/ so they can be imported
by external modules (needed for claude-pm integration).

- internal/app/ → pkg/app/
- internal/docker/ → pkg/docker/
- internal/config/ → pkg/config/
- internal/utils/ → pkg/utils/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/oxker/main.go

@@ -5,7 +5,7 @@ import (
 	"os"
 
 	tea "charm.land/bubbletea/v2"
-	"github.com/oxker/oxker/internal/app"
+	"github.com/oxker/oxker/pkg/app"
 )
 
 func main() {