Add example scripts for common operations

Create runnable example scripts demonstrating SDK usage.

**Acceptance Criteria:**
- Example: create-group.ts (create a new group)
- Example: join-and-contribute.ts (join group and make contribution)
- Example: query-group.ts (fetch and display group info)
- Add instructions in README for running examples
- Use dotenv for configuration

**Files:** new directory `examples/`, `README.md`