Previously, stripMentions() removed all @mention placeholders from
message text, including mentions of other users. This meant messages
like "@Bot create a task, assign to @张三" arrived at the agent as
"create a task, assign to" — losing the referenced user entirely.
Now only the bot's own mention is stripped. Other user mentions are
replaced with their display name (e.g. @_user_2 → @张三), so the
agent can see who was referenced in the original message.
Closes#51
sean2077
35e013c8e7