Fix help for docker-agent standalone binary exec

[gtardif]

🐠 ./bin/docker-agent --help
create or run AI agents

Usage:
  docker-agent [flags]
  docker-agent [command]

Examples:
  docker-agent run ./agent.yaml
  docker-agent run agentcatalog/pirate

(was previously displaying help for docker agent )

[docker-agent]

Review Summary

✅ Approved - No issues found in the changed code.

The logic correctly implements mutually exclusive conditions for displaying the appropriate help text:

• When running as a Docker plugin: displays docker agent syntax
• When running as a standalone binary: displays docker-agent syntax

The conditions are properly structured with !plugin.RunningStandalone() and plugin.RunningStandalone() ensuring exactly one block executes when isDockerAgent() is true.