feat: add Telegram slash command parity and menu sync

[YooooEX]

Context

Telegram currently only handles /stop and /llm, while the shared chat command surface already supports /help, /status,
/new, /restore, and /continue. This PR keeps the change radius small by limiting updates to Telegram command wiring,
shared command metadata, and README parity. No new dependencies are added.

Summary

Sync the Telegram bot command menu with the shared chat command surface and add the missing Telegram slash commands.

Changes

frontends/chatapp_common.py

• Extract shared help command metadata into HELP_COMMANDS
• Add TELEGRAM_MENU_COMMANDS for Telegram menu registration
• Add build_help_text() so help text and Telegram menu definitions stay aligned

frontends/tgapp.py

• Register Telegram bot commands on startup via ApplicationBuilder.post_init(...) and bot.set_my_commands(...)
• Route slash commands through a dedicated Telegram command handler
• Add Telegram support for /help, /status, /new, /restore, /continue, and /continue N
• Normalize bot-suffixed commands such as /help@botname
• Cancel active Telegram stream tasks before stop/reset/restore flows
• Preserve the existing streaming, HTML rendering, and file upload behavior for normal text messages

README.md

• Add Telegram to the list of frontends that support the common chat commands in both English and Chinese sections