feat(wechat): add WeChat channel plugin

[zccyman]

Summary

• Add @openclaw/wechat channel plugin for WeChat integration
• Supports text, image, and voice messages with rich outbound formatting
• Multi-account support with configurable schema
• 103 passing tests covering all core modules

What's included

• Channel plugin: Full WeChat channel implementation with WebSocket connectivity
• Message types: Text, image, voice message support
• Outbound formatting: Markdown-style formatting for rich message output
• Multi-account: Support for multiple WeChat accounts with per-account config
• Tests: 29 test files with 103 passing tests

Files

extensions/wechat/ - 29 files added

• index.ts - Plugin entry point
• src/channel.ts - WeChat channel definition
• src/bot.ts - Bot management
• src/client.ts - WeChat client wrapper
• src/outbound.ts - Message formatting
• src/send.ts - Message sending logic
• src/accounts.ts - Multi-account management
• src/runtime.ts / src/runtime-api.ts - Runtime integration
• src/config-schema.ts - Configuration schema
• src/types.ts - Type definitions
• tests/ - 17 test files