docs: Add comprehensive PROJECT_INTERVIEW_GUIDE for interviews

Add detailed interview preparation guide covering:
- 30-second elevator pitch
- Technical architecture (hexagonal + DDD)
- 16 Q&A across 5 themes (Architecture, Implementation, Testing, Challenges, Business)
- Code references for every answer
- Trade-off analysis for key decisions
- Real debugging stories (duplicate optimization bug)
- Testing strategy (407 tests breakdown)
- Connection to other Semantic Intent projects

Demonstrates senior-level engineering through:
- System design thinking
- Testing discipline
- Documentation quality
- Trade-off analysis
- Production readiness

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

Co-Authored-By: Claude <[email protected]>

Author: semanticintent