brokerd: answers carry the Mattermost post; Table::contains

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

crates/brokerd/tests/broker_pending.rs

@@ -73,6 +73,7 @@ fn answer_entry(
 fn approved() -> Answer {
     Answer::Approved {
         by: Some("bxctl".to_string()),
+        post: None,
     }
 }
 
@@ -166,6 +167,7 @@ fn a_denied_verdict_reaches_the_requester_and_nothing_runs() {
     let (mut stream, approval, _) = start(&broker, r#"{"path":"/n/a"}"#);
     let refused = Answer::Refused {
         by: Some("bxctl".to_string()),
+        post: None,
         reason: None,
     };
     answer_by_hand(&broker, approval, refused);