Verify that prompt caching is maximized

[johnsonr]

See https://platform.openai.com/docs/guides/prompt-caching

We need to ensure that everything we do--for example, system prompts--maximizes the potential for cache usage.

We should also document patterns that are important for users to consider: For example, avoiding timestamps or other unnecessarily dynamic content in PromptContributors.

[johnsonr]

Should also look at nativeUsage in Usage and potentially expose something.