Quarterly: re-check Skill install conventions

[github-actions]

Quarterly re-check of the four per-platform Skill-install conventions
hard-coded in nsc/cli/skill_commands.py _RESOLVERS. Re-read each
canonical source below and confirm the documented user-scoped path is
still current.

• claude-code — https://code.claude.com/docs/en/skills
• codex — https://developers.openai.com/codex/skills
• gemini — https://geminicli.com/docs/cli/skills/
• copilot — https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/add-skills

If any convention drifted, update all of:

• nsc/cli/skill_commands.py (the relevant _resolve_* resolver)
• docs/guides/using-with-ai-agents.md (per-target path table)
• skills/netbox-super-cli/SKILL.md (if it lists per-target paths)
• CHANGELOG.md (note the change under the unreleased section)

Close this issue once all four are confirmed (and any drift is shipped).