fix(channel): strip system tags and render Markdown for Telegram messages

[letrii]

Problem

When using ai-devkit channel start, two issues affect Telegram message quality:

1. System tags leak: Claude Code injects structured XML tags (<command-name>, <command-message>, <local-command-stdout>,
   <system-reminder>, etc.) into the conversation for the UI layer. Without filtering, these tags appear verbatim in Telegram
   messages, making responses unreadable.

2. Markdown not rendered: Claude Code responses use standard Markdown formatting (**bold**, _italic_, `code`, code
   blocks) which Telegram displays as raw text by default.

Solution

• Add stripSystemTags() in channel.ts to remove known system tags from agent output before forwarding to Telegram. Regular
  message content is preserved.
• Add markdownToHtml() in TelegramAdapter.ts to convert standard Markdown to Telegram HTML, sending messages with parse_mode: 'HTML'. Code blocks are extracted first to prevent formatting replacements inside them.

System tags stripped

• command-name
• command-message
• command-args
• local-command-stdout
• system-reminder
• user-prompt-submit-hook

[codeaholicguy]

@letrii Thanks for the contribution.

I'd recommend splitting into two PRs. Here's why:

PR 1: fix(channel): strip system tags from Telegram messages

• This is the actual bug fix
• Small, low-risk, easy to review
• Can merge immediately

PR 2: feat(channel): render Markdown as HTML in Telegram messages

• This is a feature, not a fix
• Has meaningful complexity and open issues (chunk ordering, missing link/list support, test breakage)
• Needs its own tests and more thought on edge cases

For the markdown-to-html feature specifically, the current implementation also isn't the simplest path. Consider using telegraf's utilities instead of introducing a new function for this.