Give the gonext repo itself an AGENTS.md, with CLAUDE.md as a pointer

#13 establishes that guardrail content is tool-neutral and lives once in `AGENTS.md`, with each tool's own file reduced to a pointer — but that applies to generated projects only. This repo still carries its guardrails exclusively in `CLAUDE.md`, so anyone working on gonext itself with Codex, Cursor or anything else gets nothing.

Apply the same shape here: move the content of the root `CLAUDE.md` into `AGENTS.md` and leave `CLAUDE.md` as a pointer. Consistency matters beyond convenience — it's hard to argue the pattern to users of the tool while the tool's own repo does something else.

Came out of an ideation session around #13, not a scoped request.

Start with the root CLAUDE.md and compare its guardrail content with the pattern established in issue #13. Create the root AGENTS.md with the shared content and reduce CLAUDE.md to a pointer, then verify that both files are present and the repository's instructions remain complete.

Written by the indexing model from the issue text.