chore: ai agent config

Signed-off-by: Dmytro Stanchiev <git@dmytros.dev>

.ruler/02-BUN-GUIDE.md

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+## Bun Guidelines
+
+**CRITICAL**: Do not assume you know full Bun APIs. For **ANY** Bun API you use, confirm them by using `bun-docs` MCP tools.
+
+Default to using Bun instead of Node.js.
+
+- Use `bun <file>` instead of `node <file>` or `ts-node <file>`
+- Use `bun test` instead of `jest` or `vitest`
+- Use `bun build <file.html|file.ts|file.css>` instead of `webpack` or `esbuild`
+- Use `bun install` instead of `npm install` or `yarn install` or `pnpm install`
+- Use `bun run <script>` instead of `npm run <script>` or `yarn run <script>` or `pnpm run <script>`
+- Use `bunx <package> <command>` instead of `npx <package> <command>`
+- Bun automatically loads .env, so don't use dotenv.
+
+### APIs
+
+- `Bun.serve()` supports WebSockets, HTTPS, and routes. Don't use `express`.
+- `bun:sqlite` for SQLite. Don't use `better-sqlite3`.
+- `Bun.redis` for Redis. Don't use `ioredis`.
+- `Bun.sql` for Postgres. Don't use `pg` or `postgres.js`.
+- `WebSocket` is built-in. Don't use `ws`.
+- Prefer `Bun.file` over `node:fs`'s readFile/writeFile
+- Bun.$`ls` instead of execa.
+
+### Testing
+
+#### Quick Start
+- Run tests: `bun test`
+- Write tests in `tests/` folder
+
+#### Test Structure
+- Use `describe` blocks to group related tests
+- Use `test` for individual test cases
+- Use `beforeEach`/`afterEach` for setup/teardown
+
+#### Assertions
+- Import: `import { test, expect, describe, beforeEach, afterEach, mock } from "bun:test";`
+- Common: `expect(value).toBe(expected)`, `expect(fn).rejects.toThrow()`
+- Async: `await expect(asyncFn()).resolves.toBe(expected)`
+
+#### Mocking
+- Mock functions: `mock(fn)`
+- Mock globals: `global.fetch = mock(...)`
+- Restore mocks in `afterEach` or `finally`
+
+#### Best Practices
+- Mock external APIs (fetch, file I/O)
+- Test error cases and edge conditions
+- Use descriptive test names
+- Clean up resources in `afterEach`
+
+For more information, read the Bun API docs in `node_modules/bun-types/docs/**.mdx`.