Refine preamble for general coding questions

[jtibshirani]

Follow up to #4516. In dogfooding, we noticed the new "context preamble" made Claude too cautious to answer general coding questions when other context was provided. This PR refines the prompt to strike a better balance between grounding answers in context, while still responding to general questions.

Example: "how do slices in go work?", Claude 3 Sonnet with codebase context

• Before: "Unfortunately the provided codebase context does not contain any information about how slices in Go work... I would need access to Go language documentation."
• After: "In Go, slices are reference types..."

Test plan

Added several more "Chat quality" unit tests

[jtibshirani]

Thanks for the review! I'll merge in latest main and check tests pass.