Document the builder overlay model (per-device deltas layered onto a
fresh OpenIPC/firmware clone), the build flow in builder.sh, device
directory anatomy, the package/ and devices/common/ conventions, the CI
workflows (including the moving-ref cache caveat), and a step-by-step
"Adding a new device" walkthrough.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
