chore(skills): remove builtin aionui skills

[kaizhou-lab]

Summary

• Remove the auto-injected builtin aionui-skills marketplace skill from the embedded builtin skill corpus.
• Assert /api/skills/builtin-auto no longer returns aionui-skills.
• Lower embedded auto-inject count expectations from 4 to 3 while leaving AionUi Butler skills unchanged.

Test Plan

• cargo fmt --all -- --check
• cargo test -p aionui-app --test skills_builtin_e2e
• cargo test -p aionui-extension skill_service::tests::embedded_lists_auto_inject_from_corpus -- --exact
• cargo clippy -p aionui-app -p aionui-extension -- -D warnings
• cargo test --workspace

Note: just push was started after the commit, but was interrupted at user request because the full workspace test had already completed successfully.