[#3] Add CLAUDE.md project documentation

CLAUDE.md

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+# PlotLink
+
+Collaborative on-chain storytelling platform. Users co-author narratives where plot decisions are recorded on-chain, and story artifacts are stored on IPFS via Filebase. The app is mobile-first with a terminal/monospace design aesthetic.
+
+## Tech Stack
+
+- **Framework**: Next.js 16 (App Router)
+- **Language**: TypeScript
+- **Styling**: Tailwind CSS v4
+- **Auth & Database**: Supabase
+- **Storage**: Filebase (IPFS)
+- **Chain**: EVM-compatible (RPC + contract interaction)
+
+## Repo Structure
+
+```
+src/
+  app/          # Next.js App Router pages and layouts
+    globals.css # Design tokens (CSS custom properties)
+    layout.tsx  # Root layout (monospace font)
+    page.tsx    # Home page
+.github/
+  workflows/
+    ci.yml      # Lint + type-check on PRs
+```
+
+## Commands
+
+```sh
+npm run dev        # Start dev server
+npm run build      # Production build
+npm run lint       # ESLint
+npm run typecheck  # TypeScript type-check (tsc --noEmit)
+```
+
+## Design System
+
+Terminal aesthetic: dark background (`#0a0a0a`), monospace font (Geist Mono), green accent (`#00ff88`), outline-based UI. CSS custom properties defined in `src/app/globals.css`.
+
+## Proposal
+
+The full project proposal will be added at `docs/PROPOSAL-plotlink.md` in a future ticket.
+
+## Environment Variables
+
+See [`.env.example`](.env.example) for all required environment variables.