Enable Telegram channel support in telegram plugin

[Tristan-Wilson]

Summary

Adds channel_post support to the Telegram plugin, enabling bot-to-bot communication via Telegram channels.

Telegram Bot API prevents bots from seeing other bots messages in groups — this is a hard platform limitation. However, this restriction does not apply to channels, where posts arrive as channel_post updates instead of message updates. This PR adds the necessary handlers and access control to receive and send channel posts, enabling use cases like coordinating multiple Claude Code instances via a private Telegram channel.

Change 1: Channel post handlers and access control

• Add ChannelPolicy type and channels field to Access config in access.json
• Add channel case in gate() — placed before the ctx.from check since channel posts may lack sender info
• Add channel_post:* handlers for text, photo, document, voice, audio, video (mirrors existing message:* handlers)
• Adjust handleInbound() to fall back to ctx.channelPost when ctx.message is undefined, and handle missing ctx.from
• Update assertAllowedChat() to check channels keys for outbound messages (reply, react, etc.)

Change 2: Include channel_post in polling allowed_updates

Grammy default long-polling does not subscribe to channel_post updates. Without explicitly including it in allowed_updates, the bot never receives channel posts even though the handlers are registered. This adds allowed_updates: ["message", "message_reaction", "channel_post"] to the bot.start() call.

Configuration

Add a channel to ~/.claude/channels/telegram/access.json:

{
  "channels": {
    "-100XXXXXXXXXX": { "allowFrom": [] }
  }
}

• Empty allowFrom accepts all posts in the channel (the channel being listed is sufficient authorization)
• To restrict to specific sender IDs, list them in allowFrom

Test plan

• Verified outbound: bot can send messages to a channel via reply(chat_id=...)
• Verified inbound: bot receives channel_post updates from the channel
• Verified bot-to-bot: two Claude Code instances (each with their own Telegram bot) communicating via a private channel
• Existing group and DM functionality unaffected