chore: add AGENTS.md (#730)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: danadajian

AGENTS.md

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+# Comparadise — Agent Guidelines
+
+Comparadise is a visual regression testing tool consisting of three main packages plus a shared library:
+
+- **`app/`** — Bun + React web app (tRPC backend, React Query frontend, Tailwind CSS)
+- **`action/`** — GitHub Action that runs visual tests and uploads screenshots to S3
+- **`comparadise-utils/`** — Cypress utilities for writing visual tests for web
+- **`shared/`** — Constants shared across packages
+
+## Package Manager & Monorepo
+
+Use **`bun`** exclusively. Never use `npm` or `yarn`.
+
+This is an **Nx monorepo**. Run cross-package tasks with `nx` or `bunx nx`.
+
+## Testing
+
+- **Unit tests (action):** `bunx nx test action`
+- **Unit tests (backend):** `bunx nx test backend`
+- **Unit tests (comparadise-utils):** `bunx nx test comparadise-utils`
+- **Cypress component tests (frontend):** `bunx nx test frontend`
+
+Run only the tests relevant to your changes. Do not run E2E tests or start the app.
+
+## Linting
+
+Fix lint and format issues before committing.
+
+```bash
+bun lint              # ESLint (TypeScript + React rules)
+bun format-check      # Prettier
+bun tsc               # TypeScript type check
+```
+
+## Code Conventions
+
+- TypeScript everywhere; strict mode is on via `typescript-eslint`
+- React components live in `app/frontend/`
+- Backend tRPC routes live in `app/backend/src/`
+- Shared constants (S3 directory names, image names, etc.) go in `shared/index.ts`
+- Tailwind for all styling — class ordering is enforced by `prettier-plugin-tailwindcss`
+
+## Action Build
+
+The GitHub Action runs from compiled output. After changing files in `action/src/`, rebuild:
+
+```bash
+bunx nx build action
+```
+
+Commit the updated `action/dist/` files — CI will fail if they are out of sync with source.
+
+## Key Architectural Notes
+
+- S3 stores images under `base-images/`, `new-images/`, and `original-new-images/` directories
+  - `base-images/` stores the reference images used by visual tests to compare new images against
+  - `new-images/{commit-hash}` stores the paths to `base.png`, `diff.png`, and `new.png` that represent regression for a visual test suite. The `diff.png` and `new.png` files are the diff and new images (respectively) generated by visual tests.
+  - `original-images/{commit-hash}` stores the `new.png` files corresponding to the test in `new-images/` that would become the new base images if visual changes are accepted. This is used only for usage of the resize-width and/or resize-height action inputs.