Implement Lark/Feishu as a new channel for ZeroClaw (Issue #164).
- Add LarkChannel with Channel trait impl (name, listen, send)
- listen: HTTP server (axum) for event callback with URL verification
(challenge response) and im.message.receive_v1 text message parsing
- send: POST /open-apis/im/v1/messages with tenant_access_token auth
- get_tenant_access_token with caching and auto-refresh on 401
- Allowlist filtering by open_id (same pattern as other channels)
- Add LarkConfig to schema (app_id, app_secret, verification_token, port, allowed_users)
- Register lark in channel list, doctor, and start_channels
- 18 unit tests: config serde, allowlist, channel name, message parsing,
edge cases (unicode, missing fields, invalid JSON, wrong event type)
- Fix pre-existing SchedulerConfig compile error on main
