chore: sync agent instructions with icp-cli wording

[marc0olo]

Summary

Aligns llms.txt and AGENTS.md wording with the changes made in dfinity/icp-cli#453.

• Remove "Before writing any ICP code" — too narrow (skills are useful for non-code tasks too) and redundant when AGENTS.md is loaded at conversation start
• Replace "Then fetch the full skill content from:" with "use it if already loaded in your context. Otherwise, fetch its content on-demand from the registry:" — prevents agents from redundantly re-fetching skills already in context