feat: add reply/quoted message context support

Add generic reply context fields to NewMessage (reply_to_message_id,
reply_to_message_content, reply_to_sender_name) so any channel can
pass quoted message context to the agent.

- Add thread_id and reply_to_* fields to NewMessage interface
- Add DB migration for reply context columns on messages table
- Update storeMessage/getMessagesSince/getNewMessages to persist and
  retrieve reply fields
- Render reply context as <quoted_message> XML in formatMessages
- Add DB and formatting tests

Co-Authored-By: Alfred-the-buttler <leon.alfred.bot@gmail.com>
Co-Authored-By: moktamd <moktamd@users.noreply.github.com>
Co-Authored-By: gurixs-carson <gurixs-carson@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/router.ts

@@ -16,7 +16,14 @@ export function formatMessages(
 ): string {
   const lines = messages.map((m) => {
     const displayTime = formatLocalTime(m.timestamp, timezone);
-    return `<message sender="${escapeXml(m.sender_name)}" time="${escapeXml(displayTime)}">${escapeXml(m.content)}</message>`;
+    const replyAttr = m.reply_to_message_id
+      ? ` reply_to="${escapeXml(m.reply_to_message_id)}"`
+      : '';
+    const replySnippet =
+      m.reply_to_message_content && m.reply_to_sender_name
+        ? `\n  <quoted_message from="${escapeXml(m.reply_to_sender_name)}">${escapeXml(m.reply_to_message_content)}</quoted_message>`
+        : '';
+    return `<message sender="${escapeXml(m.sender_name)}" time="${escapeXml(displayTime)}"${replyAttr}>${replySnippet}${escapeXml(m.content)}</message>`;
   });
 
   const header = `<context timezone="${escapeXml(timezone)}" />\n`;