AGENTS.md is the canonical repo guide, with CLAUDE.md a thin import

## Summary

Rename `CLAUDE.md` to `AGENTS.md` and reduce `CLAUDE.md` to an `@AGENTS.md` import plus what is true of Claude Code alone.

## Motivation

Every rule in `CLAUDE.md` is about this library, not about Claude: the tooling inventory, the "no Nix" absence note, the workflow-naming convention, and the scope discipline built on the `1.0.0.0` API-commitment milestone. Only Claude Code reads that filename, so an agent that isn't Claude Code gets none of it — including the warning that touching the module export list forces a major PVP bump.

## Scope

- `git mv CLAUDE.md AGENTS.md`, retitling the heading away from "repo guide for Claude".
- `CLAUDE.md` becomes an `@AGENTS.md` import plus only Claude-Code-specific rules, if any remain.
- Update the `CONTRIBUTING.md` pointer if it names `CLAUDE.md`.

## Acceptance criteria

- [ ] `AGENTS.md` carries the library rules
- [ ] `CLAUDE.md` imports it and states nothing another agent also needs
- [ ] The rule for adding a future tool config is written down
- [ ] Pre-commit passes

## Gotcha

This is the simplest case in the fleet: the content is wholly tool-agnostic, and there's no `.vale.ini` or `.ls-lint.yml` to update, so it may reduce to the rename plus a two-line `CLAUDE.md`. If nothing turns out to be Claude-specific, a `CLAUDE.md` holding only `@AGENTS.md` is the right answer — Claude Code won't read `AGENTS.md` without it.

Sibling Haskell repos carry the same `CLAUDE.md`-shaped guide, so whatever wording lands here is worth reusing in `collection-json.hs`.

## Additional context

Reference implementation: [dungeon-studio/genshin.dungeon.studio#1417](https://github.com/dungeon-studio/genshin.dungeon.studio/pull/1417), closing [#877](https://github.com/dungeon-studio/genshin.dungeon.studio/issues/877) there.

- [GitHub: add repository custom instructions](https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions) — Copilot's cloud agent reads `AGENTS.md`, `CLAUDE.md`, and `.github/copilot-instructions.md`.
- [Claude Code: AGENTS.md](https://code.claude.com/docs/en/memory#agents-md) — Claude Code reads only `CLAUDE.md`, and prescribes the `@AGENTS.md` import for this case.