Slack dispatches `app_mention` events (not `message` events) when a bot
is @mentioned in a channel. Without handling this event type, the bot
only responds in DMs while silently ignoring channel mentions.
Add a `*slackevents.AppMentionEvent` case to `handleEvent` with the same
validation pipeline (bot filter, old-message guard, AllowList check,
session key generation) used for regular messages.
Also fix stale go.sum checksum for github.com/BurntSushi/toml.
Closes#48
Shawn
ef37f6f7ab