docs: add AGENTS guidelines (#86)

AGENTS.md

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+# Project Overview
+
+This repo hosts a small Vue 3 + TypeScript web app built with Vite. The source lives in `src/` and is served via `index.html`. Static assets are under `public/`. Config files include `vite.config.ts` for build settings, `tsconfig.*.json` for TypeScript, and `eslint.config.mjs` for lint rules.
+
+# Guidelines
+
+- Use **semantic commit messages** (e.g. `feat:`, `fix:`, `docs:`) and the same format for PR titles.
+- Use **Yarn** for installing packages.
+- Run linting and type checks before committing any code changes.
+- Skip linting and type checks when editing only documentation or comments.
+- Follow the style rules defined by Prettier and ESLint. Prettier settings: no semicolons, single quotes, width 100, two spaces.
+- Node.js version is controlled via `.nvmrc` (22.17.0).
+
+# Linting & Formatting
+
+Install dependencies and run tools with Yarn:
+
+```bash
+yarn install  # once
+
+yarn lint        # runs ESLint with autofix
+yarn type-lint   # checks TypeScript using ts-standard
+yarn type-fix    # fixes TypeScript style issues
+
+yarn format      # formats using Prettier
+```
+
+No automated tests are defined.