docs(sdk): document run-scoped LLM headers

[bozhnyukAlex]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

• Document how to pass run-scoped LLM headers from local and remote conversations.
• Include the equivalent Agent Server request body and explain scope, precedence, persistence, and security considerations.
• Related SDK implementation: feat(sdk): support run-scoped LLM headers software-agent-sdk#4066

Validation: 34 documentation tests passed. Two unrelated pricing tests currently error because their upstream raw GitHub source returns HTTP 404.