draft: ollama: add Ollama context option

[tmc]

Summary

Adds support for Ollama's context feature to enable short conversational memory between requests. This addresses GitHub discussion #973.

Changes

• Add WithContext([]int) option function to Ollama client
• Add comprehensive example demonstrating usage patterns
• Add test coverage for the new functionality
• Add documentation explaining context usage with Ollama

Usage

// Use context from previous response for follow-up requests
llm, err := ollama.New(
    ollama.WithModel("llama2"),
    ollama.WithContext(contextFromPreviousResponse),
)

Notes

• Context enables the model to maintain state between requests
• Particularly useful for follow-up questions that reference previous context
• Context is model-specific and should only be reused with the same model
• Provides foundation for future enhancements like context extraction from responses

Test Plan

• [x] Unit tests for WithContext() option
• [x] Example code demonstrating usage patterns
• [x] Documentation coverage
• [ ] Integration tests with running Ollama server (requires manual testing)

Addresses discussion #973