docs: point agents at cli-schema from help and README

`elastic cli-schema` and `docs/cli/schema.json` are easy to miss: not on root `--help`, not in the README install path, and `cli-schema` is a reserved meta command.

Add one line to root `--help` (LEARN MORE) and a short README section: prefer `elastic --help --json` for one command, `elastic cli-schema` for the full tree. Do not tell agents to ingest the 100k-line `schema.json`.

Also add a one-line `CLAUDE.md` that is `@AGENTS.md`.

Acceptance: `elastic --help` mentions `cli-schema` and `--help --json`. README has a short "for agents" block. `CLAUDE.md` exists and only references `AGENTS.md`.