The modelByChannel feature added in the 2026.2.21 release notes causes a config schema validation failure that prevents the gateway from starting, regardless of where the key is placed.

OpenClaw version: 2026.2.21-2
OS: macOS

Steps to reproduce:

1. Add modelByChannel inside channels.discord as described in the release notes
2. Run openclaw gateway restart

Error:
channels.discord: Unrecognized key: modelByChannel

Expected:
Gateway starts and applies per-channel model overrides.

Actual:
Gateway refuses to start due to schema validation failure.