docs: enhance copilot instructions with comprehensive guidance

- Add detailed repository structure with all files and directories
- Document validation workflows and their purposes
- Add common patterns section with 7 practical examples
- Include build process explanation and flow
- Add comprehensive debugging section (local, CI, runtime)
- Expand troubleshooting table with validation failures
- Document files to avoid modifying for safety
- Add script naming conventions and example references
- Include image layers and caching best practices
- Update resources with additional documentation links
- Expand from ~600 to 936 lines for better Copilot guidance

Author: castrojo