Fix empty WRENN_TEMPLATE_ID after resuming paused sandbox

Resume() was building VMConfig without TemplateID, so Firecracker MMDS
received an empty string. envd's PostInit then wrote that empty value to
/run/wrenn/.WRENN_TEMPLATE_ID. Fix by persisting the template ID in
snapshot metadata during Pause and reading it back during Resume.

internal/snapshot/local.go

@@ -64,6 +64,7 @@ func MetaPath(baseDir, name string) string {
 // RootfsMeta records which base template a CoW file was created against.
 type RootfsMeta struct {
 	BaseTemplate string `json:"base_template"`
+	TemplateID   string `json:"template_id,omitempty"`
 }
 
 // WriteMeta writes rootfs metadata to the snapshot directory.