Add Feishu (飞书) channel plugin for enterprise messaging in China/International. Features: - WebSocket and Webhook connection modes - DM and group chat support with @mention gating - Inbound media support (images, files, PDFs) - Card render mode with syntax highlighting - Typing indicator via emoji reactions - Pairing flow for DM approval Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>