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/formatting.test.ts

@@ -115,6 +115,62 @@ describe('formatMessages', () => {
     expect(result).toContain('<messages>\n\n</messages>');
   });
 
+  it('renders reply context as quoted_message element', () => {
+    const result = formatMessages(
+      [
+        makeMsg({
+          content: 'Yes, on my way!',
+          reply_to_message_id: '42',
+          reply_to_message_content: 'Are you coming tonight?',
+          reply_to_sender_name: 'Bob',
+        }),
+      ],
+      TZ,
+    );
+    expect(result).toContain('reply_to="42"');
+    expect(result).toContain(
+      '<quoted_message from="Bob">Are you coming tonight?</quoted_message>',
+    );
+    expect(result).toContain('Yes, on my way!</message>');
+  });
+
+  it('omits reply attributes when no reply context', () => {
+    const result = formatMessages([makeMsg()], TZ);
+    expect(result).not.toContain('reply_to');
+    expect(result).not.toContain('quoted_message');
+  });
+
+  it('omits quoted_message when content is missing but id is present', () => {
+    const result = formatMessages(
+      [
+        makeMsg({
+          reply_to_message_id: '42',
+          reply_to_sender_name: 'Bob',
+        }),
+      ],
+      TZ,
+    );
+    expect(result).toContain('reply_to="42"');
+    expect(result).not.toContain('quoted_message');
+  });
+
+  it('escapes special characters in reply context', () => {
+    const result = formatMessages(
+      [
+        makeMsg({
+          reply_to_message_id: '1',
+          reply_to_message_content: '<script>alert("xss")</script>',
+          reply_to_sender_name: 'A & B',
+        }),
+      ],
+      TZ,
+    );
+    expect(result).toContain('from="A &amp; B"');
+    expect(result).toContain(
+      '&lt;script&gt;alert(&quot;xss&quot;)&lt;/script&gt;',
+    );
+  });
+
   it('converts timestamps to local time for given timezone', () => {
     // 2024-01-01T18:30:00Z in America/New_York (EST) = 1:30 PM
     const result = formatMessages(