Expose executor skills through skill tools

[copyberry]

Expose executor skills through skill tools

What changed

• Add executor authority support to skills.list and skills.read for skills from selected capability roots.
• Allow skills.read to load package-relative resources referenced by an executor skill while keeping reads within the selected package.
• Paginate skill listings and resource contents with bounded response sizes and stale-cursor validation.
• Resolve executor filesystems per sampling step so skill tools use the active selected capability roots.

Testing

• Extend the app-server executor skill test to list an executor skill, read its main SKILL.md, and read a paginated reference resource.
• Cover visibility filtering, metadata bounds, pagination, invalid cursors, package boundaries, and resource size limits.