Document installing the skills with lk skills

[bcherry]

What does this PR do?

Documents the lk skills install path. It was split out of #11 so #11 could merge before lk skills ships.

Merge after livekit/livekit-cli#996 ships in a CLI release.

• README: adds a LiveKit CLI install section (lk skills install) ahead of the plugin, and tells Claude Code users to pick either the plugin or lk skills.
• AGENTS.md: lists the LiveKit CLI as a distribution channel, and adds lk skills to the tools that track a skill by its lock-file hash.

Related: livekit/web#6370 carries the matching lk skills docs on the docs site.

Checklist

• Content passes the "freeze forever" test: no flag rosters, schema listings, API surfaces, or version numbers; --help and the docs are referenced instead
• No time-relative phrasing ("newer CLIs", "as of writing", "is landing")
• N/A: descriptions. No skill description changed.
• uv run evals/validate.py passes
• N/A: trigger eval. No skill description changed.
• N/A: output eval. No skill body changed.

Eval results

n/a: no description or body changes. README and AGENTS.md only.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GhopX4p6rY3MNkNQ1M832x

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