Add support for AI model selection through CLI or environment variables

[uday-rana]

Is your feature request related to a problem? Please describe. AI model support is limited to llama3-8b-8192.

Describe the solution you'd like Add support for users to specify the AI model to use by:

• Providing a --model or -m flag in the command-line interface.
• Enabling selection through an environment variable, such as AI_MODEL.

If no model is specified, a default should be set.

Describe alternatives you've considered N/A

Additional context N/A

[uday-rana]

Partially addressed by https://github.com/uday-rana/codeshift/commit/bfa5043cd0d5d16f152377966641ec68510fadfc