feat: add ProxyStream wire types for agent-relayed console

proto/vantage/v1/vantage.proto

@@ -14,6 +14,7 @@ service Vantage {
   rpc ReportChecks(ReportChecksRequest)         returns (ReportChecksResponse);
   // Bidirectional stream: agent sends auth once, server pushes commands.
   rpc CommandStream(stream AgentMessage)        returns (stream ServerCommand);
+  rpc ProxyStream(stream ProxyClientMsg) returns (stream ProxyServerMsg);
 }
 
 message RegisterRequest {
@@ -180,6 +181,7 @@ message ServerCommand {
     ApplyUpdatesCmd apply_updates = 5;
     RunStepCmd      run_step      = 6;
     CleanupWorkspaceCmd cleanup_workspace = 7;
+    OpenProxyCmd open_proxy = 8;
   }
 }
 
@@ -228,3 +230,33 @@ message StepOutputChunk {
   bytes  data       = 3;
   bool   eof        = 4;
 }
+
+// OpenProxyCmd tells the agent to dial 127.0.0.1:port locally and relay that
+// connection back over a fresh ProxyStream identified by proxy_id.
+message OpenProxyCmd {
+  string proxy_id = 1;
+  uint32 port     = 2;
+}
+
+message ProxyOpen {
+  string server_id   = 1;
+  string agent_token = 2;
+  string proxy_id    = 3;
+}
+
+message ProxyClose { string reason = 1; }
+
+message ProxyClientMsg {
+  oneof payload {
+    ProxyOpen  open  = 1;   // first message only
+    bytes      data  = 2;
+    ProxyClose close = 3;
+  }
+}
+
+message ProxyServerMsg {
+  oneof payload {
+    bytes      data  = 1;
+    ProxyClose close = 2;
+  }
+}