feat: improve resolver error detection and documentation (#41)

Add comprehensive troubleshooting guide and better error messages for resolver export issues.

## Changes

### Documentation (README.md)
- Add detailed troubleshooting section for "RollupError: not exported" errors
- Include common resolver export mistakes with examples
- Explain how resolver scanning works (oxc-parser)
- Add debugging steps for resolver issues
- Add Claude Code setup prompts for Nuxt and Nitro

### Resolver Scanning (src/utils/index.ts)
- Detect and warn about default exports vs named exports
- Warn when named exports don't use define functions
- Warn when resolver files have no valid exports
- Add try-catch to handle parse errors gracefully
- Show which exports were found but not registered
- All warnings only shown in development mode

### Development Experience (src/index.ts)
- Add startup diagnostics showing resolver count
- Show breakdown of resolver types (query, mutation, etc.)
- Validate resolver setup on dev server start
- Provide immediate feedback on GraphQL configuration

## Benefits
- Users get clear error messages when resolvers aren't detected
- Better debugging experience with detailed warnings
- Comprehensive documentation reduces support requests
- Helps users migrate from default to named exports

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: productdevbook