Add GRUB to top-level AGENTS.md and README

Update installer patch description in AGENTS.md to mention GRUB
alongside ext4. Add redbear-full-grub build target and GRUB CLI
section to README.

AGENTS.md

@@ -180,7 +180,7 @@ local/patches/
 ├── base/redox.patch                # Applied to base source during build (symlinked from recipe)
 ├── base/P0-*.patch                 # Individual logical patches
 ├── relibc/P3-*.patch               # POSIX gap patches (eventfd, signalfd, timerfd, etc.)
-├── installer/redox.patch           # Installer ext4 support
+├── installer/redox.patch           # Installer ext4 + GRUB bootloader support
 └── build-system/
     ├── 001-rebrand-and-build.patch # Makefile, mk/*, scripts, build.sh rebranding
     ├── 002-cookbook-fixes.patch    # src/ Rust fixes (fetch.rs, staged_pkg.rs, repo.rs, html.rs)

README.md

@@ -156,10 +156,22 @@ Requires a Linux x86_64 host with Rust nightly, QEMU, and standard build tools.
 ```bash
 make all CONFIG_NAME=redbear-full        # Full desktop + custom drivers
 make all CONFIG_NAME=redbear-minimal     # Minimal server
-make live CONFIG_NAME=redbear-full       # Live ISO (redbear-live.iso)
+make all CONFIG_NAME=redbear-full-grub   # Full desktop with GRUB boot manager
+make live CONFIG_NAME=redbear-live       # Live install ISO (redbear-live.iso)
+make live CONFIG_NAME=redbear-live-mini  # Tiny boot-test live ISO (~256 MiB image)
 make qemu                                # Boot in QEMU
 ```
 
+### GRUB Boot Manager (optional)
+
+Red Bear OS can use GNU GRUB as an alternative boot manager with Linux-compatible CLI:
+
+```bash
+make all CONFIG_NAME=redbear-full-grub   # Build with GRUB chainload
+local/scripts/grub-install --target=x86_64-efi --disk-image=build/x86_64/harddrive.img
+local/scripts/grub-mkconfig -o local/recipes/core/grub/grub.cfg
+```
+
 ## Native hardware listing tools
 
 Red Bear configs now include a small native `redbear-hwutils` package that ships `lspci` and