- Add SLACK_THREAD_REQUIRE_MENTION, SLACK_IGNORE_OTHER_USER_MENTIONS and
SLACK_REQUIRE_MENTION_CHANNELS to the conftest behavioral-env force-clear
list so config-loader side effects can't leak between tests (same class
of leak the existing SLACK_* entries guard against).
- Document thread_require_mention and require_mention_channels in the
Slack messaging guide next to the other mention-gating options.