Include missed messages when catching up the agent

When a triggered message comes in, fetch all messages in that chat
since the last agent interaction and include them in the prompt.
Each message is formatted with timestamp and sender.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/db.ts

@@ -72,3 +72,13 @@ export function getNewMessages(jids: string[], lastTimestamp: string): { message
 
   return { messages: rows, newTimestamp };
 }
+
+export function getMessagesSince(chatJid: string, sinceTimestamp: string): NewMessage[] {
+  const sql = `
+    SELECT id, chat_jid, sender, content, timestamp
+    FROM messages
+    WHERE chat_jid = ? AND timestamp > ?
+    ORDER BY timestamp
+  `;
+  return db.prepare(sql).all(chatJid, sinceTimestamp) as NewMessage[];
+}