Document project-wide extra_instructions in shared llm config

[richardkiene]

Summary

Clarifies that extra_instructions can be set at the shared llm: config level for project-wide instructions that apply to both judge and synthetic_user.

Changes

• Add use case: "Setting project-wide guidelines that apply to all test runs"
• Clarify that shared config applies to both judge and synthetic_user
• Add example showing project-wide instructions in llm: section
• Add example showing combined shared + component-specific instructions
• Clarify that per-scenario instructions are additive