Adds a strict_mention config option that, when enabled, requires an
explicit @-mention on every message in channel threads. Disables the
'once mentioned, forever in the thread' and session-presence auto-triggers.
- New _slack_strict_mention() helper (config.extra + SLACK_STRICT_MENTION env)
- Bridged top-level slack.strict_mention yaml to SLACK_STRICT_MENTION env,
matching require_mention/allow_bots bridging
- Unit tests for the helper + config bridge