Enhance prompt processing with existing file content support

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Overview

This PR improves the prompt-based file generation functionality by allowing the OpenAI-assisted file generation process to consider and modify existing file content when applicable.

Changes

• Added support for passing existing file content to the OpenAI prompt within file_item.py.
• Updated default OpenAI model to gpt-4.1 from gpt-3.5-turbo.
• Modified generate.py to read existing file content and pass it to the process_prompt() method.
• Created a new example configuration file example/gpt.yaml demonstrating usage with user prompts.

Justification

This change provides more intelligent and context-aware file generation by allowing existing content to be passed into the prompt. It helps to enhance productivity and avoid overwriting relevant content unintentionally.

Impact

• Improves integration with OpenAI by enabling content-aware generation.
• Prevents accidental data loss when regenerating files.
• Demonstrates usage with an example YAML file, aiding users in understanding configuration.