feat(mcp): prompts/instructions support

[dwmkerr]

Summary

Add support for MCP prompts/instructions to the mock MCP server.

Context

Part of the follow-on features mentioned in #36. MCP supports prompts/instructions that can be used to provide guidance to AI systems.

Resources

• MCP Prompts Specification

Acceptance Criteria

• Implement prompts/list method
• Implement prompts/get method
• Add configurable prompts via mock-llm.yaml
• Add tests for prompts functionality
• Update documentation
• Add sample demonstrating prompts usage