feat(mattermost): add mention_only config for @-mention filtering

Add mention_only support for the Mattermost channel, matching the existing
Discord implementation. When enabled, the bot only processes messages that
contain an @-mention of the bot username, reducing noise in busy channels.

- Add mention_only field to MattermostConfig schema (Option<bool>, default false)
- Rename get_bot_user_id() to get_bot_identity() returning (user_id, username)
- Add contains_bot_mention_mm() with case-insensitive word-boundary matching
  and metadata.mentions array support
- Add normalize_mattermost_content() to strip @-mentions from processed text
- Wire mention_only through channel and cron factory constructors
- Add 23 new tests covering mention detection, stripping, case-insensitivity,
  word boundaries, metadata mentions, empty-after-strip, and disabled passthrough

src/config/schema.rs

@@ -1573,6 +1573,10 @@ pub struct MattermostConfig {
     /// When false, replies go to the channel root.
     #[serde(default)]
     pub thread_replies: Option<bool>,
+    /// When true, only respond to messages that @-mention the bot.
+    /// Other messages in the channel are silently ignored.
+    #[serde(default)]
+    pub mention_only: Option<bool>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]