feat: support non-hidden sgptrc config file in .config/shell_gpt

[mariano-soto]

Summary

This PR adds support for a non-hidden configuration file sgptrc inside .config/shell_gpt/, as requested in #751, in addition to the existing .sgptrc.

Motivation

Currently, ShellGPT reads configuration from .config/shell_gpt/.sgptrc. While functional, using a hidden file inside an already hidden directory (.config) feels redundant and slightly unintuitive.

Supporting sgptrc aligns better with common conventions in .config directories and improves developer experience.

Changes

• Added support for .config/shell_gpt/sgptrc
• Implemented fallback logic to preserve backward compatibility
• Ensured no breaking changes to existing behavior

Behavior

Configuration file resolution now follows this priority order:

1. .config/shell_gpt/sgptrc
2. .config/shell_gpt/.sgptrc (fallback)

Testing

Added tests to verify:

• sgptrc is used when present
• .sgptrc is used when sgptrc is absent

All tests pass and linters succeed.

Backward Compatibility

This change is fully backward compatible. Existing users relying on .sgptrc will continue to work without modification.

Future Improvements (optional)

• Emit a deprecation warning when .sgptrc is used

Closes #751