chore: add AGENTS.md

AGENTS.md

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+# AGENTS.md
+
+## Build/Lint/Test Commands
+- **Install deps**: `npm ci` (workspace-aware, installs all packages)
+- **Build all**: `npm run build` (Turbo orchestrates all workspace builds)
+- **Dev server**: `npm run dev` (starts all services in development mode)
+- **Type check**: `npm run type-check` (Vue app: `vue-tsc`, backend: `tsc`)
+- **Lint**: `npm run lint` (uses Prettier + Tailwind plugin)
+- **Run single test**: `npx vitest run path/to/file.test.ts` (add Vitest to sync-client first)
+- **Backend migrations**: `npm run migrations:gen` && `npm run migrations:push`
+
+## Code Style Guidelines
+- **Imports**: Group third-party, internal workspace, and relative imports with blank lines
+- **Formatting**: Prettier with Tailwind plugin, 2-space indentation, single quotes, trailing commas
+- **Types**: Explicit TypeScript types, avoid `any`, use `Record<string, unknown>` for generic objects
+- **Naming**: `camelCase` for variables/functions, `PascalCase` for classes/types, `UPPER_SNAKE_CASE` for constants
+- **Error handling**: Wrap async DB/network calls in try/catch, use custom error classes, log with Sentry
+- **Async patterns**: Prefer `await` over fire-and-forget IIFEs, ensure errors are caught
+- **Vue**: Use `<script setup lang="ts">`, Composition API, Pinia for state management
+- **Backend**: tRPC for type-safe APIs, Drizzle ORM, Lucia for auth
+- **Sync protocol**: Always include `protocolVersion` in payloads, reject mismatches with errors
+
+## Notes
+- Monorepo with Turbo for orchestration
+- No existing Cursor/Copilot rules found
+- Sync-client needs test framework setup (Vitest recommended)