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

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

crates/brokerd/tests/ledger_answer.rs

@@ -33,6 +33,7 @@ fn pending(ledger: &Ledger) -> Ask {
 fn bxctl() -> Answer {
     Answer::Approved {
         by: Some("bxctl".to_string()),
+        post: None,
     }
 }
 
@@ -206,6 +207,7 @@ fn a_refusal_is_recorded_with_the_owners_reason() {
     let ask = pending(&ledger);
     let refused = Answer::Refused {
         by: Some("bxctl".to_string()),
+        post: None,
         reason: Some("not now".to_string()),
     };
     let answered = ledger.answer(ask, 0, refused, &asking(), now());