Add redbear-full-grub config with GRUB boot manager

Includes bootloader = "grub" and efi_partition_size = 16. Builds GRUB
as primary boot manager that chainloads the Redox bootloader.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

config/redbear-full-grub.toml

@@ -0,0 +1,14 @@
+# Red Bear OS Full Configuration with GRUB Boot Manager
+# Desktop + RBOS branding + GRUB chainload
+#
+# Build: make all CONFIG_NAME=redbear-full-grub
+#
+# This config adds GRUB as boot manager (Phase 2 installer-native).
+# GRUB presents a menu and chainloads the Redox bootloader.
+# Requires: efi_partition_size >= 8 (16 recommended for GRUB + Redox bootloader + margin)
+
+include = ["redbear-full.toml"]
+
+[general]
+bootloader = "grub"
+efi_partition_size = 16