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/channels/mod.rs

@@ -1301,6 +1301,7 @@ pub async fn start_channels(config: Config) -> Result<()> {
             mm.channel_id.clone(),
             mm.allowed_users.clone(),
             mm.thread_replies.unwrap_or(true),
+            mm.mention_only.unwrap_or(false),
         )));
     }