[MaikoTan]

Adds AGENT.md, a reference doc for AI coding agents working on this repo.

Contents:

• Project overview — what the extension does (schema validation, koishi.yml IntelliSense, snippets)
• Repository structure — annotated tree of src/, schemata/, snippets/, scripts/, .github/workflows/, plus a note that schemata/*.json, snippets/*.json, out/ and *.vsix are generated and git-ignored (edit the .yaml sources instead)
• Toolchain — Node 20+, pinned Yarn 4 via Corepack, TypeScript, yaml, ESLint, @vscode/test-electron, vsce
• Commands — full package.json script table, then explicit Build / Test / Package / Debug sections
• Architecture notes — the shared cursorAt → SchemaIndex resolution pipeline and its non-obvious gotchas (untyped-key fallback via indentation, empty value slots must read as value positions, per-request re-parsing)
• Contribution guidelines — YAML-first authoring, yarn test + yarn package are CI gates, tag-driven releases

Kept alongside the existing Chinese-language AGENTS.md; this one is in English to match the rest of the repo's public docs.

Verified locally: yarn run compile, yarn run convert, and yarn run lint all pass.