feat(mattermost): add thread_replies config and typing indicator

Add two Mattermost channel enhancements:

1. thread_replies config option (default: false)
   - When false, replies go to the channel root instead of threading.
   - When true, replies thread on the original post.
   - Existing thread replies always stay in-thread regardless of setting.

2. Typing indicator (start_typing/stop_typing)
   - Implements the Channel trait's typing methods for Mattermost.
   - Fires POST /api/v4/users/me/typing every 4s in a background task.
   - Supports parent_id for threaded typing indicators.
   - Aborts cleanly on stop_typing via JoinHandle.

Updated all MattermostChannel::new call sites (start_channels, scheduler)
and added 9 unit tests covering thread routing and edge cases.

src/config/schema.rs

@@ -1499,6 +1499,10 @@ pub struct MattermostConfig {
     pub channel_id: Option<String>,
     #[serde(default)]
     pub allowed_users: Vec<String>,
+    /// When true, replies thread on the original post. When false (default),
+    /// replies go to the channel root.
+    #[serde(default)]
+    pub thread_replies: Option<bool>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]