fix: prevent full message history from being sent to container agents

When lastAgentTimestamp was missing (new group, corrupted state, or
startup recovery), the empty-string fallback caused getMessagesSince to
return up to 200 messages — the entire group history. This sent a
massive prompt to the container agent instead of just recent messages.

Fix: recover the cursor from the last bot reply timestamp in the DB
(proof of what we already processed), and cap all prompt queries to a
configurable MAX_MESSAGES_PER_PROMPT (default 10). Covers all three
call sites: processGroupMessages, the piping path, and
recoverPendingMessages.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/db.ts

@@ -375,6 +375,19 @@ export function getMessagesSince(
     .all(chatJid, sinceTimestamp, `${botPrefix}:%`, limit) as NewMessage[];
 }
 
+export function getLastBotMessageTimestamp(
+  chatJid: string,
+  botPrefix: string,
+): string | undefined {
+  const row = db
+    .prepare(
+      `SELECT MAX(timestamp) as ts FROM messages
+       WHERE chat_jid = ? AND (is_bot_message = 1 OR content LIKE ?)`,
+    )
+    .get(chatJid, `${botPrefix}:%`) as { ts: string | null } | undefined;
+  return row?.ts ?? undefined;
+}
+
 export function createTask(
   task: Omit<ScheduledTask, 'last_run' | 'last_result'>,
 ): void {