feat(telegram): add Post message rendered as HTML with tags and buttons

[m1ngsama]

First step of #56: plugins need one message type that renders as a designed post on Telegram and as their existing output everywhere else.

• New Post message part: Markdown body, an optional lazily built image, hashtags, up to 3 link buttons, and a silent flag.
• Telegram renders it with sendPhoto or sendMessage in HTML. The body supports bold, italic, code, links, lists and quotes. Headings render as bold. Captions are cut at 1024 characters and texts at 4096. Link previews are off.
• If Telegram rejects the HTML, the post is resent as plain text.
• OneBot and Satori send Post.Fallback, so QQ output stays byte-for-byte the same.
• Telegram recall and group rename now work in forum topics. They passed chat:topic as the chat id.
• QQ comment pushes link the issue again, as before feat(github-bridge): send cards with link buttons instead of rich text #54.

No plugin sends a Post yet.