Revise README and improve untracked dependency handling

Streamlines and clarifies the README with concise instructions, improved troubleshooting, and updated dependency tracking guidance. Enhances cpp-library's CMake install logic to distinguish between development and install modes for untracked dependencies: now, configuration emits warnings and uses fallbacks for local builds, but installation fails with a clear error if dependencies are not properly tracked. Adds install-time validation to prevent broken package configs and updates user guidance for mapping and tracking dependencies.

Author: sean-parent