Make wp-coding-agents skills opt-in

[chubes4]

Summary

• Keep installing the ongoing upgrade-wp-coding-agents skill by default, while ensuring the one-shot setup skill is never installed onto target runtimes.
• Preserve unmanaged/user skills instead of deleting anything outside the wp-coding-agents-owned upgrade skill.
• Collapse setup interview/verification branches into plain markdown files referenced by the setup skill, instead of exposing them as separate fragmented skills.

Testing

• tests/post-upgrade-restore.sh
• tests/bridge-render.sh
• bash -n setup.sh upgrade.sh lib/skills.sh bridges/kimaki/post-upgrade.sh lib/summary.sh tests/post-upgrade-restore.sh

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the implementation, ran targeted verification, and prepared the PR summary for Chris to review.