mom: add [SILENT] response marker for periodic events

When mom responds with just [SILENT], the status message is deleted
and nothing is posted to Slack. Useful for periodic events that check
for activity but find nothing to report.

packages/mom/src/main.ts

@@ -207,6 +207,16 @@ function createSlackContext(event: SlackEvent, slack: SlackBot, state: ChannelSt
 			});
 			await updatePromise;
 		},
+
+		deleteMessage: async () => {
+			updatePromise = updatePromise.then(async () => {
+				if (messageTs) {
+					await slack.deleteMessage(event.channel, messageTs);
+					messageTs = null;
+				}
+			});
+			await updatePromise;
+		},
 	};
 }