Add support for including external libraries in context

Implemented a new configuration option external_library_paths to specify paths to external libraries.
Updated CodeContext to accept and process external library paths.
Enhanced the include command to handle external libraries.
Modified the sample command to ensure external libraries are included in sampling.
Added validation for absolute paths to ensure they exist and are correctly formatted.
Updated documentation to reflect the new configuration option and usage examples.Add support for including external libraries in context
Implemented a new configuration option external_library_paths to specify paths to external libraries.
Updated CodeContext to accept and process external library paths.
Enhanced the include command to handle external libraries.
Modified the sample command to ensure external libraries are included in sampling.
Added validation for absolute paths to ensure they exist and are correctly formatted.
Updated documentation to reflect the new configuration option and usage examples.---

The setup script failed. You can see its log here.

Please give me instructions on how to fix it or push commits to fix it yourself.

Closes #23

jakethekoenig / mentat