When a member-started thread root @mentions the assignee agent, replies
in that thread should trigger on_comment — the thread is a conversation
with the agent, not a member-to-member chat.
Previously isReplyToMemberThread only checked the reply content for
assignee mentions. Now it also checks the parent (thread root) content.
This fixes a gap where path 1 (on_comment) suppressed the trigger and
path 2 (on_mention) skipped the assignee, leaving no trigger path.
fc6405e4be