fix(vibe): save .env API key file to project root instead of CWD

[Ananya-Kaushal137]

Summary

This PR fixes an issue where the vibe command would save the user's OpenAI API key to a .env file in the current working directory instead of the project root. This effectively resolves the problem of "lost" API keys when running the command from a subdirectory.

Closes #1258

Context

During the interactive mofa vibe setup, if an OpenAI API key is missing, it prompts the user to enter and save it. Previously, the saving logic used os.path.join(os.getcwd(), '.env') while the loading logic correctly used the project root.
If a user ran mofa vibe from a subdirectory (e.g., examples/), the key was written locally to that subdirectory. Subsequent runs from other directories would fail to locate the key, frustrating the user and continuously prompting for setup. This change unifies the file path logic.

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Changes

• Replaced the hardcoded os.getcwd() path in mofa/commands/vibe.py with the existing _get_env_file_path() helper.
• Ensures API keys are always appended or saved to the project root's .env file consistently.

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How you Tested

1. Set a mock project_root and navigated to a different subdirectory (e.g., cd /path/to/project/subdirectory).
2. Cleared any existing OPENAI_API_KEY from the environment.
3. Invoked the mofa vibe CLI.
4. Entered a mock API key when prompted and selected Y to save it to .env.
5. Verified that the .env file was correctly generated in the project root, and no .env file was created in the immediate working directory.

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