New docs/_CUB_RBPKGBUILD_IMPL_PLAN.md with full CUB package builder specification
covering RBPKGBUILD format, CLI commands, build flow, BUR repository, and AUR conversion.
Updated AGENTS.md with pkgutils extensions and CUB integration details.
Updated AMD GPU integration docs with current P2 progress.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
