chore: add CLAUDE.md and Copilot code review instructions

[nameless-mc]

Why

The repository root has no CLAUDE.md (only website/CLAUDE.md, which is scoped to the Docusaurus docs), and there are no GitHub Copilot code review instructions. This PR adds AI agent files with a clear role split:

• Claude: implementation support — repository layout, toolchain, commands, and testing policy in CLAUDE.md
• Copilot: PR review — review rules in .github/copilot-instructions.md and path-scoped .github/instructions/*.instructions.md

What

• CLAUDE.md — implementation context for Claude (layout, toolchain, commands, testing policy, cross-repo dependencies)
• .github/copilot-instructions.md — repo-wide Copilot review rules (correctness / security / code quality / tests / review style)
• .github/instructions/typescript.instructions.md — TypeScript review rules (applyTo: "src/**/*.ts")
• .github/instructions/tests.instructions.md — vitest / Cucumber review rules (applyTo: "src/**/__tests__/**,features/**")
• .claude/rules/*.md — Claude-side loaders that @import the matching Copilot file with paths: frontmatter, so Claude applies the same rules during implementation without duplicating content. The files under .github/ are the canonical source.

How to test

• Documentation-only change; no runtime code is affected.
• pnpm lint:prettier passes.
• Verified each Copilot instruction file stays under the 4,000-character read limit (wc -m): copilot-instructions.md 3,117 / typescript.instructions.md 2,675 / tests.instructions.md 2,347.
• Copilot review behavior can be confirmed on subsequent PRs once this is merged.

Checklist

• Read CONTRIBUTING.md
• Updated documentation if it is required.
• Added/updated tests if it is required. (or tested manually)
• Passed pnpm lint and pnpm test on the root directory.

[Copilot]

Pull request overview

Adds repo-wide AI agent guidance files so Claude has implementation context (CLAUDE.md) and GitHub Copilot has consistent PR review instructions (repo-wide + path-scoped rules), with Claude loaders that import the canonical Copilot rules to avoid duplication.

Changes:

• Added root CLAUDE.md describing repository layout, toolchain, commands, testing policy, and cross-repo dependencies.
• Added GitHub Copilot review instructions (.github/copilot-instructions.md) plus path-scoped rule files for TypeScript and tests.
• Added .claude/rules/*.md loader files that scope and import the matching Copilot instruction files.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
CLAUDE.md	Adds root-level implementation context (layout/tooling/commands/testing policy) and links to AI instruction files.
.github/instructions/typescript.instructions.md	Adds TypeScript-specific PR review rules for src/**/*.ts.
.github/instructions/tests.instructions.md	Adds test review rules intended for vitest + Cucumber paths.
.github/copilot-instructions.md	Adds repo-wide Copilot PR review priorities (correctness/security/tests/review style).
.claude/rules/typescript.md	Claude loader importing the canonical Copilot TypeScript rules for src/**/*.ts.
.claude/rules/tests.md	Claude loader importing the canonical Copilot test rules for tests/features paths.
.claude/rules/repo-wide.md	Claude loader importing the canonical repo-wide Copilot rules.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.