[User Story] As a user, I want a leet answer command so that I can answer programming questions

**Given...**
An open question exists.

**When...**
Any user sends the message "c!leetans `title`; response:
\```lang
`response`
\```".

**Then...**
Query the answer against the targeted question on [leetcode](https://leetcode.com/problemset/all/), and return whether the answer was correct or not.

Start with the requested `c!leetans` message format and the linked LeetCode problem set. The issue does not name implementation files, tests, or an existing command entry point, so those must be located in the bot before work begins. Done means the command targets the requested question, evaluates the submitted response, and reports whether it is correct.

由索引模型根据 Issue 内容生成。