- Added a Go scheduler plugin demonstrating CLIProxyAPI capabilities, such as `plugin.register`, `plugin.reconfigure`, and `scheduler.pick`.
- Implemented methods for plugin configuration, built-in scheduler delegation (`fill-first`, `round-robin`), dynamic candidate selection, and error handling.
- Extended `pluginhost` with scheduler handling, candidate normalization, and fallback mechanisms.
- Included examples, tests, and detailed documentation for scheduler usage and implementation.
