[Bug]: Per-agent installation does not work for universal agents

Description

It's not possible to install skills only for specific agent if it's flagged as universal

Steps to Reproduce

1. Run npx skills add example --agent opencode

Expected Behavior

Skill is installed at .config/opencode/skills

Actual Behavior

Skill is installed at .agents/skills

Version

1.7.6

Node.js Version

No response

Operating System

Linux

Logs / Error Output

undefined