Upload all skills from a .zip archive

Upload every skill found inside a .zip archive in a single request.

Expected zip structure:

skills.zip
├── 01-cinematic/
│   └── SKILL.md
├── 07-ecommerce-ad/
│   └── SKILL.md
└── ...

Each sub-folder must contain a SKILL.md with valid YAML frontmatter.

Returns a summary with skill_id and latest_version per skill, plus error details for any failures.

Set skip_existing=true to safely re-run without creating duplicates.