Fix unbootable installs: lay the kernel into the target and own GRUB

archiso keeps vmlinuz/initramfs in the ISO boot dir, not the squashfs, so
unpackfs lays down an empty /boot. The chroot's mkinitcpio/grub-mkconfig had
nothing to work with and the ESP ended up empty (firmware found no bootloader).

- shellprocess@kernel (dontChroot) copies the live kernel into the target
  /boot before the bootloader step
- post-install.sh now runs grub-install itself, including a --removable pass
  so firmware with no NVRAM entry still boots via EFI/BOOT/BOOTX64.EFI

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/airootfs/etc/calamares/modules/shellprocess-kernel.conf

@@ -0,0 +1,8 @@
+---
+# Lay the kernel into the target /boot before the bootloader/initramfs steps.
+# Runs in the live environment (not the chroot) so it can read the ISO boot dir.
+dontChroot: true
+timeout: 60
+
+script:
+    - "/usr/bin/bash /usr/local/bin/bos-copy-kernel ${ROOT}"