Load the full instructions for a named skill.

Use this after calling `list_skills` when a user request matches one of the discovered skills. Call this before starting work that the skill covers.

Provide:
- name: exact skill name returned by `list_skills`.

The response includes the full skill instructions wrapped in <skill-instructions> tags, the skill directory path, and a list of supporting resource files. Load resource files on demand using available file tools when the skill instructions reference them.
