feat(cli): improve CLI agent and command handling

[KaiveYoung]

This PR adds --base-url (-b) CLI flag support for the Roo-Code CLI, enabling users to connect to OpenAI-compatible APIs and custom API endpoints. It also adds openai as a supported provider.

Key changes:

• Added -b, --base-url <url> CLI option to specify a custom base URL for LLM providers
• Added openai to the list of supported providers (anthropic, openai-native, openai, gemini, openrouter, vercel-ai-gateway)
• Updated getProviderSettings() in provider.ts to handle baseUrl for all applicable providers:
  • openai: openAiBaseUrl, openAiApiKey, openAiModelId
  • openai-native: openAiNativeBaseUrl
  • gemini: googleGeminiBaseUrl
  • openrouter: openRouterBaseUrl
• Updated apps/cli/README.md with the new flag documentation

Design choices:

• The baseUrl parameter is passed through the ExtensionHostOptions interface to maintain consistency with existing provider settings
• Default providers map to their respective base URL configurations, while custom URLs can be overridden via the CLI flag

Test Procedure

1. Lint check: Run pnpm lint from the project root - all checks pass
2. CLI flag test:

   # Test with OpenAI-compatible API
   cd apps/cli && pnpm start -b https://api.openai.com/v1 -k $OPENAI_API_KEY --provider openai -m gpt-4o
   
   # Test with local LLM server (e.g., Ollama)
   cd apps/cli && pnpm start -b http://localhost:11434/v1 --provider openai -m llama3