When a top-level comment @mentions an agent (non-assignee), subsequent
replies in the same thread now also trigger that agent via on_mention.
Previously only the current comment's mentions were checked, so replies
without an explicit re-mention would silently skip the agent.
Extends enqueueMentionedAgentTasks to accept the parent comment and
merge its parsed mentions (deduplicated) into the trigger set, reusing
all existing guards (self-trigger, assignee skip, visibility, dedup).
Closes MUL-177
c9c8230271