OpenClaw Discord messaging plugin is missing from sandbox image

[sandl99]

Summary

OpenClaw sandboxes with Discord messaging enabled report that the official Discord external plugin is missing.

Observed

Inside a sandbox:

sandbox@40342a00be54:~$ openclaw channels status --probe
...
Gateway not reachable; showing config-only status.
Config: /sandbox/.openclaw/openclaw.json
Mode: local

- openclaw-weixin 9d3256b0797f-im-bot: enabled, configured

Missing official external plugins:
- Discord: Install the official external plugin with: openclaw plugins install @openclaw/discord, or run: openclaw doctor --fix.

Discord is configured by NemoClaw, but the required OpenClaw Discord plugin is not installed in the sandbox image/config surface.

Expected

When Discord is selected for the OpenClaw agent, NemoClaw should ensure the official Discord external plugin is installed or staged so OpenClaw can load it without requiring a manual openclaw plugins install @openclaw/discord after sandbox creation.

Acceptance Criteria

• OpenClaw Discord messaging sandboxes include the required official Discord plugin installation/staging.
• openclaw channels status --probe does not report Discord as a missing official external plugin after NemoClaw onboarding/rebuild.
• Rebuild preserves the Discord plugin installation.
• Add coverage for the OpenClaw Discord channel config/plugin installation path.