feat: Add OpenAI support for plugin generation

[MikeBirdTech]

• Add OpenAI as a supported AI service option with GPT-5 as default model
• Update AIService class to handle OpenAI model configuration
• Update CLI to accept --ai openai option
• Update README with OpenAI setup instructions and usage examples

Fixes #1

🤖 Generated with Claude Code

[mentatbot]

I'm getting to work. See my steps and track my cost here.

• Wake on any new activity.
  _{I always see new activity while actively working. When paused, I'll only respond when tagged unless this box is checked. Learn more about triggers.}

[mentatbot]

The OpenAI integration looks good overall! I tested the functionality and confirmed that:

✅ OpenAI is properly added to the CLI options
✅ Default model (gpt-5) is set correctly
✅ Custom models can be specified and work as expected
✅ README documentation is updated appropriately

However, I found two issues that should be addressed:

1. Ollama service type bug: There's a mismatch between the CLI option (--ai ollama) and the service type check in the query method, which would cause ollama requests to not work properly.

2. ASCII art warning: The ASCII art string contains invalid escape sequences that generate SyntaxWarnings.

Both issues have simple fixes suggested above.