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

config/redbear-full-grub.toml

@@ -12,3 +12,8 @@ include = ["redbear-full.toml"]
 [general]
 bootloader = "grub"
 efi_partition_size = 16
+
+# GRUB recipe must be built before the installer runs.
+# Including it here ensures `make all` builds it as part of the normal package set.
+[packages]
+grub = {}