feat: add server: false mode and simplify external services config (#60)

- Add `server: false` option to disable GraphQL server functionality
  - Skip schema, resolver, and directive scanning
  - Skip route handler registration
  - Skip server type generation
  - Skip server scaffold files
  - Still allows external services client types and SDK generation

- Simplify external services configuration
  - Make `schema` optional (defaults to `endpoint`)
  - Auto-enable `downloadSchema` for URL schemas
  - Reduce required config from 4 fields to just 2 (name + endpoint)

- Fix hot reload for Nitro framework client directory
  - Add client directory to watchDirs for Nitro projects

- Add dev:reload hook for client-only mode
  - Ensures file changes trigger client type regeneration

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Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: productdevbook