When requireMention is true for a group, thread replies still require
an explicit @mention — even though thread context makes the intent
unambiguous. This adds a requireMentionInThread config option at both
the global and per-group level, letting users opt out of the mention
requirement for thread replies while keeping it for top-level group
messages.
Closes#40475
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>