core: add configurable <context_window_guidance> message

[bolinfest]

Why

This PR adds a configurable <context_window_guidance> developer section immediately after <context_window>. Harness integrations need this section to give the model deployment-specific instructions for preparing for context-window transitions.

What changed

• Add an optional features.token_budget.guidance_message config with a 1,000-byte runtime cap and generated schema support.
• Render configured guidance as a developer ContextualUserFragment wrapped in <context_window_guidance> immediately after <context_window>.
• Omit the section when guidance is unset, empty, or whitespace-only.
• Preserve the resolved value in config locks and classify persisted guidance as contextual developer content.
• Add integration coverage for rendered content and ordering.