AGENTS.md is the canonical agent 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

The file already calls itself an agent guide in its own heading, and its content matches: the tooling inventory, the rule that the Collection+JSON spec beats the test suite, the Haddock and comment conventions, the PVP change classification, the release process, and the don't-touch list. None of it is Claude-specific, yet only Claude Code reads that filename. An agent that isn't Claude Code could shift semantics to make a failing test pass, or ship an upper-bound widening as a major bump, with none of these rules loaded.

## Scope

- `git mv CLAUDE.md AGENTS.md`.
- `CLAUDE.md` becomes an `@AGENTS.md` import plus only Claude-Code-specific rules, if any remain.
- Add `AGENTS.md` to `.vale.ini` so it lints under the rules `CLAUDE.md` gets today.

## Acceptance criteria

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

## Gotcha

The heading already reads "collection-json — agent guide", so this is close to a pure rename. The one line to re-examine is the Scope discipline pointer to the `issue-work` skill: that skill is a host-local Claude Code artifact, not something another agent can invoke, so it either moves to `CLAUDE.md` or gets restated as the rule it encodes.

Vale styles here are vendored under `.vale/styles` with no `vale sync`, so the new path needs adding to `.vale.ini` by hand.

## 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.