ssh-agent plugin should honor existing SSH agent

[FallenWarrior2k]

If the feature request is for a plugin or theme, specify it here.

ssh-agent plugin

If the feature solves a problem you have, specify it here.

If an SSH agent is already configured, it is rarely desirable to start another agent. A common scenario for this is using GNOME Keyring's support for the SSH agent, in which case there will already be a running agent instance that is implicitly shared across all processes that will ever be started in the session, which also means the plugin's cache mechanism isn't needed.

Describe the proposed feature.

Don't start a new agent instance if there is already one running, i.e. SSH_AUTH_SOCK is set and valid.

Describe alternatives you've considered

Currently, I set zstyle :omz:plugins:ssh-agent agent-forwarding yes, which coincidentally also works by checking SSH_AUTH_SOCK before starting a new agent. However, I feel like this is abusing a control knob originally intended for something else.

Additional context

No response

Related Issues

No response