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/slack.rs

@@ -54,11 +54,15 @@ impl Channel for SlackChannel {
     }
 
     async fn send(&self, message: &SendMessage) -> anyhow::Result<()> {
-        let body = serde_json::json!({
+        let mut body = serde_json::json!({
             "channel": message.recipient,
             "text": message.content
         });
 
+        if let Some(ref ts) = message.thread_ts {
+            body["thread_ts"] = serde_json::json!(ts);
+        }
+
         let resp = self
             .http_client()
             .post("https://slack.com/api/chat.postMessage")
@@ -170,6 +174,7 @@ impl Channel for SlackChannel {
                             .duration_since(std::time::UNIX_EPOCH)
                             .unwrap_or_default()
                             .as_secs(),
+                        thread_ts: Some(ts.to_string()),
                     };
 
                     if tx.send(channel_msg).await.is_err() {