docs: add CONTRIBUTING.md with development and testing instructions

CONTRIBUTING.md

@@ -0,0 +1,39 @@
+# Contributing
+
+## Development Setup
+
+```bash
+npm install
+```
+
+## Building
+
+```bash
+npm run build          # Type-check + production build
+npm run dev            # Watch mode (rebuilds on file changes)
+npm run build:local    # Build + copy to Obsidian vault (requires .env with VAULT_PATH)
+```
+
+For `build:local`, create a `.env` file in the project root:
+
+```
+VAULT_PATH=/path/to/your/obsidian/vault
+```
+
+## Testing
+
+Tests use [vitest](https://vitest.dev/) and cover the pure link-building functions in `src/linkBuilder.ts`.
+
+```bash
+npm test               # Run all tests once
+npm run test:watch     # Run tests in watch mode (re-runs on file changes)
+```
+
+Test files live alongside source files with a `.test.ts` suffix (e.g., `src/linkBuilder.test.ts`).
+
+## Linting
+
+```bash
+npm run lint           # Check for lint errors
+npm run lint:fix       # Auto-fix lint errors
+```