proto: gateway, the four messages between brokerd and gatewayd

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/proto/src/wire.rs

@@ -1,5 +1,6 @@
 use serde::{Deserialize, Serialize};
 
+use crate::gateway::{ApprovalOutcome, ApprovalPosted, ApprovalReply, ApprovalRequest};
 use crate::{CallId, DataClass, DecisionRecord, SessionId, Timestamp, Usage};
 
 pub const PROTOCOL_VERSION: u32 = 1;
@@ -35,6 +36,10 @@ pub enum Message {
     Ok(Empty),
     CheckGrants(Empty),
     GrantsReport(GrantsReport),
+    ApprovalRequest(ApprovalRequest),
+    ApprovalPosted(ApprovalPosted),
+    ApprovalReply(ApprovalReply),
+    ApprovalOutcome(ApprovalOutcome),
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]