docs: add README with screenshot

README.md

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+# GroupThink
+
+> LLM-powered tab grouping for AI-first browsers.
+
+GroupThink is a Chrome extension that uses Claude (or local Ollama models) to organize your open tabs by **what you're doing**, not where you are. A YouTube video about React goes in the "Frontend Dev" group, not "YouTube." 64 tabs become 6 meaningful workspaces.
+
+![GroupThink treemap view — 64 tabs organized into semantic groups](docs/screenshots/treemap-overview.png)
+
+## How it works
+
+1. Press **Alt+Shift+G** (or **Cmd+Shift+K** on Mac) to open GroupThink
+2. The extension reads every open tab's title and URL
+3. An LLM classifies tabs by topic and intent — grouping semantically, never by domain
+4. Results render as an interactive **zoomable d3 treemap** — tiles sized by tab count, color-coded by category
+5. Adjust the **specificity slider** (1-10) to control granularity: broad workspace-level groups at 1, fine-grained subgroups at 10
+6. **Refine via chat**: type "merge News and Media" or "split React into Components and State" and the LLM restructures on the fly
+
+## Features
+
+- **Semantic grouping** — groups by topic/intent, not by website. "Billing" instead of "AWS + Stripe + QuickBooks"
+- **Zoomable treemap** — click into a group to see subgroups and individual tabs. Breadcrumb navigation back out
+- **Tab previews** — hover any tab tile to see a thumbnail screenshot and metadata
+- **Natural language refinement** — chat bar at the bottom for multi-turn restructuring
+- **Specificity control** — slider from broad (3-5 groups) to fine-grained (nested subgroups for 4+ tabs)
+- **Dual LLM support** — Anthropic Claude (cloud) or Ollama (local/free: qwen2.5, llama3.1, mistral, gemma2)
+- **Browser context enrichment** — optionally uses visit history, bookmarks, and recent sessions for smarter classification
+- **Cache-first loading** — shows cached grouping instantly; regroups only on explicit refresh
+- **Keyboard shortcut** — Alt+Shift+G / Cmd+Shift+K to open from anywhere
+
+## Architecture
+
+```
+popup (browser action)
+  └── opens app/index.html in new tab
+
+app (React)
+  ├── TreemapView         zoomable d3 treemap visualization
+  │   ├── TreemapNode     recursive tile renderer (groups, children, tabs)
+  │   ├── TabPreview      hover popover with screenshot thumbnail
+  │   └── TreemapBreadcrumb  hierarchy navigation
+  ├── SpecificitySlider   1–10 granularity control
+  ├── ChatBar             natural language refinement input
+  └── TabChaos            loading animation (tabs fly into groups)
+
+background (service worker)
+  ├── ai.ts               LLM client (Anthropic + Ollama)
+  ├── grouping.ts         post-LLM enforcement, keyword clustering, singleton merging
+  ├── prompts.ts          system prompt + per-call prompt builders
+  ├── browser-context.ts  visit history, bookmarks, top sites
+  └── tabs.ts             Chrome tabs API wrappers
+```
+
+All LLM calls happen in the background service worker. The app communicates via `chrome.runtime.sendMessage()` — `ai.ts` is never imported in UI code.
+
+## Tech stack
+
+| Layer | Technology |
+|-------|-----------|
+| UI | React 18, d3-hierarchy (treemap layout) |
+| Build | Vite 5, @crxjs/vite-plugin (Chrome MV3) |
+| AI | Anthropic SDK (Claude), Ollama (local models) |
+| Validation | Zod |
+| Language | TypeScript |
+| Linting | Biome |
+| Testing | Vitest |
+
+## Getting started
+
+**Prerequisites:** Node >= 20, pnpm 8+, Chrome
+
+```bash
+pnpm install
+
+# Configure LLM provider
+cp env.json.example env.json
+# Add your Anthropic API key, or configure Ollama URL
+
+pnpm build
+```
+
+Load the extension:
+1. Navigate to `chrome://extensions`
+2. Enable **Developer mode**
+3. Click **Load unpacked** and select the `dist/` directory
+
+After code changes: `pnpm build`, then click the reload button on the extension card.
+
+## Settings
+
+Open the extension options page to configure:
+
+- **LLM Provider** — Anthropic (cloud) or Ollama (local/free)
+- **Model** — Claude Sonnet (default), Claude Opus, Claude Haiku, or local models (qwen2.5, llama3.1, mistral, gemma2)
+- **Context enrichment** — Off (title + URL only), Basic (+ visit frequency), Full (+ bookmarks, sessions)
+- **Theme** — Light, Dark, or Auto (system preference)
+
+## License
+
+MIT