Fix docker-agent version for cli plugin & standalone exec

[gtardif]

Another quick one to cleanup version output when executing docker-agent cli plugin (standalone or plugin mode)

🐠 ./bin/docker-agent version
docker-agent version dev
Commit: 19ac4a68d80104161bf411c9ca69c16fe929c5fa
🐠 docker agent version
docker agent version dev
Commit: 19ac4a68d80104161bf411c9ca69c16fe929c5fa
🐠 ./bin/cagent version
cagent version dev
Commit: 19ac4a68d80104161bf411c9ca69c16fe929c5fa

[docker-agent]

Review Summary

✅ No issues found - The code changes look good!

This PR correctly fixes the version command output for different execution modes:

• Standalone execution (./bin/docker-agent version): Shows "docker-agent version dev" ✓
• Plugin mode (docker agent version): Shows "docker agent version dev" ✓
• cagent mode (./bin/cagent version): Shows "cagent version dev" ✓

The logic properly handles the command name construction by:

1. Using the parent command's name when available (which is set to "agent" in plugin mode, "docker-agent" in standalone)
2. Only prepending "docker " when running in plugin mode (!plugin.RunningStandalone())
3. This prevents duplicate prefixes and ensures correct output in all scenarios

The implementation is consistent with how the root command is configured in root.go (lines 102-115).