feat: Add Doubleagent - specialized AgentReady development agent (#30)

Create comprehensive Claude Code agent specification for AgentReady
repository development. The agent provides:

- Deep knowledge of AgentReady architecture and design patterns
- Understanding of assessment workflow and scoring algorithm
- Guidance for implementing and expanding assessors
- Test structure and coverage requirements
- Common development patterns and best practices
- Constitutional principles (library-first, graceful degradation)
- Performance optimization techniques
- Anti-patterns to avoid

The agent helps with:
- Implementing new assessors
- Enhancing existing assessors
- Writing comprehensive tests
- Debugging assessment issues
- Improving report templates
- Optimizing performance
- Expanding stub assessors

References key design documents:
- CLAUDE.md, BACKLOG.md
- specs/001-agentready-scorer/plan.md and research.md
- agent-ready-codebase-attributes.md
- contracts/ schemas

Implements coldstart-prompts/09-agentready-repository-agent.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: jeremyeder