The Slack docs document `slack.mention_patterns` as custom wake words that
trigger the bot alongside `@mention`, and the config layer bridges the key into
the Slack adapter's `config.extra` — but the adapter never read it. With
`require_mention` on, a channel message containing a configured wake word (and
no literal `<@BOTUID>`) was silently ignored. Every other adapter that
documents `mention_patterns` (Telegram, DingTalk, Mattermost, WhatsApp,
BlueBubbles, Photon) implements it; Slack was the odd one out.
Add `_slack_mention_patterns()` (compiled, cached; reads `slack.mention_patterns`
as a list/string or `SLACK_MENTION_PATTERNS` as a JSON/CSV/newline list, invalid
regexes warned and skipped) and `_slack_message_matches_mention_patterns()`,
mirroring the existing adapters. Channel mention detection now also triggers on
a wake-word match, so the documented field works as described.
Adds tests for pattern compilation (list/string/env/invalid-regex) and for the
channel-trigger gating with a wake word under require_mention.
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