feat: add Google Chat channel support

[vanducng]

Summary

Add Google Chat as a new channel type in the channels system, enabling GoClaw agents to communicate via Google Chat spaces.

Details

Implement a Google Chat channel integration following the existing channel architecture in internal/channels/. This should support:

• Google Chat API integration — handle incoming messages via webhook or push subscription
• Thread management — map Google Chat threads to GoClaw sessions
• Message formatting — convert LLM markdown output to Google Chat card/text format
• Space support — work in both DMs and spaces (group chats)
• Authentication — Google Service Account credentials for Bot API access

Reference

• Existing channel implementations: Telegram, Slack, Discord, Feishu/Lark, Zalo, WhatsApp
• Channel interface: internal/channels/
• Google Chat API docs: https://developers.google.com/workspace/chat/api/reference/rest

Acceptance Criteria

• New channel type googlechat registered in channel manager
• Incoming message handling (webhook/push)
• Outgoing message delivery with proper formatting
• Thread-to-session mapping
• Configuration via channel_instances table
• Documentation for setup and configuration