Bug Description

Doctor detects modelByChannel in the config and attempts to auto-enable it as a plugin, but modelByChannel is a core feature in the channels schema, not a plugin.

Error Message

[gateway] failed to persist plugin auto-enable changes: Error: Config validation failed: plugins.entries.modelByChannel: plugin not found: modelByChannel

Steps to Reproduce

1. Configure modelByChannel in channels section:

"channels": {
  "modelByChannel": {
    "whatsapp": { "120363...@g.us": "anthropic/claude-sonnet-4-6" }
  }
}

2. Start gateway
3. Doctor shows "modelByChannel configured, enabled automatically" and attempts to persist it to plugins.entries
4. Validation fails because modelByChannel is not a plugin

Expected Behavior

Doctor should recognize that modelByChannel is a core channels configuration option (defined in src/config/types.channels.ts) and not attempt to add it to plugins.entries.

Environment

• OpenClaw version: v2026.2.21 (commit bfd712db8)
• OS: macOS (arm64)

Impact

Low — gateway starts and works correctly, modelByChannel functions as expected. This is a cosmetic warning that may confuse users.

AI-assisted issue report (lightly tested, reproduced locally)