Add customizable prompts and per-scenario LLM config overrides (#52)

[richardkiene]

Summary

Add the ability to customize judge and synthetic user behavior through:

• extra_instructions that augment the default prompts
• Per-scenario LLM configuration overrides
• Separate LLM configs for judge and synthetic_user

Configuration Priority (highest to lowest)

1. CLI arguments
2. Scenario-level config (from scenario YAML)
3. Component-specific config (judge:, synthetic_user:)
4. Shared LLM config (llm:)
5. Defaults

Note: extra_instructions are appended from all levels (not replaced).

Example Usage

Global Config (mcprobe.yaml)

llm:
  provider: openai
  model: gpt-4o

judge:
  model: gpt-4o-mini  # Use cheaper model for judging
  extra_instructions: |
    Be lenient about formatting differences.
    Consider partial credit for directionally correct answers.

synthetic_user:
  model: gpt-4o
  extra_instructions: |
    Push back firmly on vague answers.

Per-Scenario Override

name: Complex Financial Analysis
description: Tests multi-step calculations

config:
  judge:
    model: gpt-4o  # Need smarter model for this one
    extra_instructions: |
      This scenario requires exact numerical precision.
  synthetic_user:
    extra_instructions: |
      You are a CFO who expects exact figures.

synthetic_user:
  persona: A CFO reviewing quarterly reports
  initial_query: What was our Q3 revenue growth?

evaluation:
  correctness_criteria:
    - Provides exact revenue figures

Changes

• Added extra_instructions field to LLMConfig model
• Added ScenarioLLMOverride and ScenarioConfig to scenario model
• Added optional config section to TestScenario
• Updated resolve_llm_config to handle scenario-level overrides
• Updated prompt builders to append extra_instructions
• Updated pytest plugin, MCP server, and CLI to pass scenario configs
• Added LLMDefaults dataclass to avoid too-many-arguments lint issue

Test plan

• All 255 unit tests pass
• Ruff linting passes
• Mypy type checking passes
• Manual testing with scenario overrides

Closes #52