Fix GRUB reassessment findings: clean-build gap, validation, robustness

- Add grub package to redbear-full-grub.toml so make all works from
  a clean tree (the installer needs grub.efi before it runs)
- Fix stat -f%z (macOS-only) to stat -c%s (Linux) in GRUB recipe
- Normalize bootloader config value to lowercase in install_inner so
  bootloader = "GRUB" from config files is accepted
- Add bad-cluster marker (0x0FFFFFF7) check in fat_tool.py
  _next_cluster to prevent potential infinite loops on degraded media
- Fix file handle leak in fat_tool.py if _read_bpb raises
- Clean up temp directory in fetch_bootloaders on error
- Update AGENTS.md with GRUB recipe and installer documentation
- Update GRUB plan with clean-build prerequisite note

local/recipes/core/grub/recipe.toml

@@ -117,7 +117,7 @@ if [ ! -f "${COOKBOOK_STAGE}/usr/lib/boot/grub.efi" ]; then
     exit 1
 fi
 
-GRUB_SIZE=$(stat -f%z "${COOKBOOK_STAGE}/usr/lib/boot/grub.efi" 2>/dev/null || stat -c%s "${COOKBOOK_STAGE}/usr/lib/boot/grub.efi" 2>/dev/null || echo "unknown")
+GRUB_SIZE=$(stat -c%s "${COOKBOOK_STAGE}/usr/lib/boot/grub.efi" 2>/dev/null || echo "unknown")
 echo "GRUB EFI image size: ${GRUB_SIZE} bytes"
 
 # Copy default GRUB configuration from recipe directory.