feat: Add support for bubblewrap sandbox

sandbox/platform/platform_linux.go

@@ -4,13 +4,11 @@
 package platform
 
 import (
-	"errors"
-
 	"github.com/safedep/pmg/sandbox"
 )
 
 // NewSandbox creates a platform-specific sandbox instance for Linux.
-// TODO: Implement Bubblewrap or seccomp-bpf based sandbox.
+// Uses Bubblewrap (bwrap) for filesystem, network, and process isolation.
 func NewSandbox() (sandbox.Sandbox, error) {
-	return nil, errors.New("sandbox not yet implemented for Linux")
+	return newBubblewrapSandbox()
 }