feat(channels): add threading support to message channels

Add optional thread_ts field to ChannelMessage and SendMessage for
platform-specific threading (e.g. Slack threads, Discord threads).

- ChannelMessage.thread_ts captures incoming thread context
- SendMessage.thread_ts propagates thread context to replies
- SendMessage::in_thread() builder for fluent API
- Slack: send with thread_ts, capture ts from incoming messages
- All reply paths in runtime preserve thread context via in_thread()
- All other channels initialize thread_ts: None (forward-compatible)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/gateway/mod.rs

@@ -1450,6 +1450,7 @@ mod tests {
             content: "hello".into(),
             channel: "whatsapp".into(),
             timestamp: 1,
+            thread_ts: None,
         };
 
         let key = whatsapp_memory_key(&msg);