Fix Codex command-skill installation

[STRUDSO]

Summary

• include user-invocable command-skills in Codex skill filtering
• mirror the same command-skill inclusion for OpenCode skill filtering
• add regression coverage proving orphan command-skills like nw-refactor install while non-command orphan skills stay filtered

Verification

• PYTHONPATH=/Users/soren.mahon@m10s.io/src/nWave .venv/bin/python -m pytest tests/installer/unit/plugins/test_codex_skills_plugin.py tests/installer/unit/plugins/test_opencode_skills_plugin.py
• isolated source installer run confirmed /tmp/nwave-codex-test/.agents/skills/nw-refactor/SKILL.md is created

Notes

• Latest main already contains nWave/skills/nw-refactor/SKILL.md; the bug was that Codex/OpenCode public filtering excluded command-skills without agent ownership.