Fix GRUB bootloader lookup, eliminate duplicate recipe, harden recipe build

install-grub.sh now searches both recipes/core/bootloader/target and
local/recipes/core/bootloader/target for the Redox bootloader artifact.

The WIP grub recipe (recipes/wip/services/grub) is now a full directory
symlink to local/recipes/core/grub instead of just recipe.toml, ensuring
COOKBOOK_RECIPE resolves to a directory that contains grub.cfg. This also
eliminates the duplicate recipe warning from the cookbook.

The GRUB recipe now fails hard (exit 1) if grub.cfg is missing instead of
just warning.

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

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

recipes/wip/services/grub

@@ -0,0 +1 @@
+../../../../local/recipes/core/grub

recipes/wip/services/grub/recipe.toml

@@ -1 +0,0 @@
-../../../../local/recipes/core/grub/recipe.toml