fix: skip invalid ssh agent sockets

[binaricat]

Summary

• ignore invalid Unix ssh-agent sockets before adding agent auth to the fallback chain
• reuse the same agent availability check in the main SSH bridge
• avoid terminal and SFTP failing early with "Failed to connect to agent" when SSH_AUTH_SOCK is stale

Testing

• node -e "require('./electron/bridges/sshAuthHelper.cjs'); console.log('sshAuthHelper ok')"
• node -e "require('./electron/bridges/sshBridge.cjs'); console.log('sshBridge ok')"
• simulated a missing SSH_AUTH_SOCK and verified auth fallback no longer returns an agent

Refs #290