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/channels/email_channel.rs

@@ -443,6 +443,7 @@ impl EmailChannel {
                 content: email.content,
                 channel: "email".to_string(),
                 timestamp: email.timestamp,
+                thread_ts: None,
             };
 
             if tx.send(msg).await.is_err() {