Add CUB implementation plan spec and update project documentation

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>

local/AGENTS.md

@@ -71,13 +71,13 @@ redox-master/                  ← git pull updates mainline Redox
 │   │   ├── core/              ← ext4d (ext4 filesystem scheme daemon + mkfs tool)
 │   │   ├── branding/          ← redbear-release (os-release, hostname, motd)
 │   │   ├── drivers/           ← redox-driver-sys, linux-kpi
-│   │   ├── gpu/               ← redox-drm (AMD + Intel display drivers)
+│   │   ├── gpu/               ← redox-drm (AMD + Intel display drivers), amdgpu (C port)
 │   │   ├── system/            ← evdevd, udev-shim, firmware-loader, redbear-meta
 │   │   ├── wayland/           ← Wayland compositor (Phase 4)
 │   │   └── kde/               ← KDE Plasma (Phase 6)
 │   ├── patches/
 │   │   ├── kernel/            ← Kernel patches (ACPI, x2APIC)
-│   │   ├── base/              ← Base patches (acpid fixes, power methods)
+│   │   ├── base/              ← Base patches (acpid fixes, power methods, pcid /config endpoint)
 │   │   ├── relibc/            ← relibc patches (POSIX: eventfd, signalfd, timerfd)
 │   │   ├── bootloader/        ← Bootloader patches
 │   │   └── installer/         ← Installer patches (ext4 filesystem support)
@@ -224,7 +224,7 @@ local/Assets/
 | loading background.png | Boot splash | Convert to framebuffer-compatible format, display before orbital starts |
 | loading background.png | Login screen | Set as orblogin/orbital background |
 
-**Current status**: Assets are committed to git. Not yet integrated into the build — requires bootloader and display server changes (Phase 2+).
+**Current status**: Assets are committed to git. Not yet integrated into the build — requires bootloader and display server integration (P2 hardware validation).
 
 ## ANTI-PATTERNS