Update llms.txt skills discovery instructions

[marc0olo]

Context

The ICP skills registry (dfinity/icskills) is migrating to v0.2 of the Agent Skills Discovery RFC. See dfinity/icskills#137 for full details.

We've observed agents failing to fetch skills because they construct wrong URLs from the {name}/{file} template in our current instructions. The fix: the skills index.json now includes a url field per skill, so agents follow a direct link instead of constructing URLs.

Current instructions (in icp-cli docs llms.txt)

## Agent skills

Tested implementation patterns for ICP development are available as agent skills.
Fetch the skills index and remember each skill's name and description:
https://skills.internetcomputer.org/.well-known/skills/index.json

When a task matches a skill's description, use it if already loaded in your
context. Otherwise, fetch its content on-demand from the registry:
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.

Recommended replacement

## Agent skills

Tested implementation patterns for ICP development are available as agent skills.

1. Fetch the skills index and remember each skill's name, description, and url:
   https://skills.internetcomputer.org/.well-known/agent-skills/index.json

2. When a task matches a skill\'s description, use it if already loaded in your
   context. Otherwise, fetch the skill content from its url.
   Do not construct skill URLs manually.

   Example: for the skill named "internet-identity", its url is:
   https://skills.internetcomputer.org/.well-known/agent-skills/internet-identity/SKILL.md

   If a response contains HTML instead of markdown, the URL is wrong.

3. 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.

Key changes

1. Remove the URL template ({name}/{file}) — this caused agents to guess wrong filenames
2. Direct agents to the url field in the index — no URL construction needed
3. Update path from /.well-known/skills/ to /.well-known/agent-skills/ (v0.2 spec change)

Note: Wait for dfinity/icskills#137 to be completed before updating the path. The url field is already available in the current index.