feat(docs): add agent skills discovery instructions to llms.txt

[marc0olo]

Summary

• Replace the single-line skills registry link in llms.txt with structured instructions that tell agents how to discover, fetch, and prioritize IC development skills
• Mirrors the pattern from dfinity/developer-docs@35a3939

Generated llms.txt now includes:

## Agent skills

Tested implementation patterns for ICP development are available as agent skills.
Before writing any ICP code, fetch the skills index and remember the skill names and descriptions:
https://skills.internetcomputer.org/.well-known/skills/index.json

When a task matches a skill description, fetch its content on-demand from:
https://skills.internetcomputer.org/.well-known/skills/{name}/{file}

Skills contain correct dependency versions, configuration formats, and common pitfalls that prevent build failures.
Always prefer skill guidance over general documentation when both cover the same topic.