docs: add comprehensive CLAUDE.md for AI assistant guidance

Add detailed documentation to help AI assistants understand and work
with the AgentAPI codebase. Includes:

- Project overview and architecture
- Detailed component descriptions (termexec, msgfmt, screentracker, httpapi)
- Development setup and workflows
- Code conventions and patterns
- Testing strategy and examples
- Common tasks and troubleshooting
- CI/CD and release process
- Quick reference guide

This documentation covers all 9 supported agents, build processes,
API endpoints, and provides practical examples for common development
tasks.

Author: claude