docs: split README into separate docs

Move tech stack, getting started, and gameplay sections into docs/
folder. Slim README down to a landing page with links.

docs/tech-stack.md

@@ -0,0 +1,29 @@
+# Tech Stack
+
+| Category | Tool | Version | Purpose |
+|----------|------|---------|---------|
+| **Runtime** | [Matter.js](https://brm.io/matter-js/) | ^0.20.0 | 2D rigid-body physics engine |
+| **Rendering** | Canvas2D | (built-in) | Fruit drawing, UI overlay, game-over screen |
+| **Build** | [Vite](https://vitejs.dev/) | ^8.0.4 | Dev server, ES module bundling, production builds |
+| **Testing** | [Vitest](https://vitest.dev/) | ^4.1.4 | Unit and integration tests |
+| **Language** | JavaScript (ES Modules) | — | Vanilla JS, no framework |
+| **Dev Tooling** | [ClaudeKit](https://github.com/carlrannaberg/claudekit) | — | Git hooks for linting, type-checking, testing, self-review |
+
+## Project Structure
+
+```
+src/
+  main.js          Entry point: initializes game, attaches event listeners
+  game.js          Game class: orchestrates engine, renderer, input, state
+  physics.js       Matter.js engine setup, world configuration, walls
+  renderer.js      Canvas2D rendering: fruits, walls, UI overlay
+  input.js         Mouse/touch input handling, drop control
+  fruits.js        Fruit definitions (sizes, colors, labels, progression)
+  merger.js        Collision detection callback, merge logic, scoring
+  constants.js     Game dimensions, physics tuning, timing constants
+  style.css        Page styling
+  *.test.js        Vitest test files
+index.html         Single HTML page with canvas element
+specs/             Feature and bugfix specifications
+docs/              Documentation
+```