feat(discord): add mention_only config for @-mention trigger (#529)

When mention_only is true, the bot only responds to messages that
@-mention the bot. Other messages in the guild are silently ignored.
Also strips the bot mention from content before processing.

Co-authored-by: Will Sarg <12886992+willsarg@users.noreply.github.com>

src/onboard/wizard.rs

@@ -2586,6 +2586,7 @@ fn setup_channels() -> Result<ChannelsConfig> {
                     guild_id: if guild.is_empty() { None } else { Some(guild) },
                     allowed_users,
                     listen_to_bots: false,
+                    mention_only: false,
                 });
             }
             2 => {