* Add Custom Agent Foundry agent
- Adds expert agent for designing and creating VS Code custom agents
- Provides comprehensive guidance on agent architecture and best practices
- Includes tool selection strategies, workflow patterns, and quality guidelines
- Updates README.agents.md with new entry
* Update README to enhance instructions for Custom Agent Foundry
* Refactor README to improve clarity and organization of Custom Agents section
* Add model specification for Custom Agent Foundry
