Fix docker ai shellout to cagent

[gtardif]

detect when cagent (standalone invocation) is invoked by a cli plugin, and run cagent standalone, not as a cli plugin

[docker-agent]

Review Summary

✅ APPROVED - No issues found.

This PR adds a straightforward fix to detect when cagent is invoked standalone by the Docker AI CLI plugin. The change adds an additional condition rootCmd.Name() == "cagent" to the existing plugin.RunningStandalone() check, which correctly handles the edge case described in the PR description.

The logic is sound:

• When docker ai shells out to cagent, the environment variables indicate it's running as a plugin, but cagent should still run in standalone mode
• The OR condition ensures both scenarios are covered
• No nil pointer risks, logic errors, or other issues identified

The code change is minimal, focused, and achieves the stated goal without introducing new vulnerabilities.