Add support for configuring and using Ollama models with Foyle:

[jlewi]

• Add 'AgentConfig' struct to 'config' package with a 'Model' field for specifying the model to use for completions
• Set default configuration for the 'AgentConfig' model to 'openai.GPT3Dot5Turbo0125'
• Update 'agent' package to handle missing AgentConfig in configuration and log an error
• Update 'oai' package to handle missing OpenAI configuration and custom BaseURL, and log BaseURL usage
• Add 'DefaultModel' constant for Ollama model name to 'const.go' in 'config' package
• Add a manual test for Ollama usage with 'Test_Ollama' in 'client_test.go' with a check for 'GITHUB_ACTIONS' environment variable
• Add documentation for using Ollama with Foyle in 'docs/content/en/docs/ollama/_index.md'
• Include steps for configuring Foyle to use Ollama's baseURL and model in the documentation

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