refactor(lifecycle): drop dead instance_port persistence entry

`instance_port` is only used as a zap log field — it's never written to
the executor metadata map, so carrying it across resume in
`persistentMetadataKeys` was a no-op.

apps/backend/internal/agent/lifecycle/executor_backend.go

@@ -86,7 +86,6 @@ var persistentMetadataKeys = map[string]bool{
 	"sprite_state":      true,
 	"sprite_created_at": true,
 	"local_port":        true,
-	"instance_port":     true,
 
 	// Executor type marker
 	MetadataKeyIsRemote: true,