chore(dankclaw): track the host GRUB command line with an installer

/etc/default/grub was host-only state while the IOMMU boot line was the most
consequential kernel decision on the box (the 2026-09-11 amd_iommu=off adoption
lived only in the decision record). Fetched byte-exact (1581 B) and paired with
scripts/set-dankclaw-bootline.sh: refuses any host but dankclaw, shows the diff,
installs 0644 root:root, runs update-grub, names the pending token, and reboots
only on --reboot. The deployer plans nothing under dankclaw/host/ by design —
kernel state is not a container restart.

dankclaw/host/grub

@@ -0,0 +1,40 @@
+# If you change this file or any /etc/default/grub.d/*.cfg file,
+# run 'update-grub' afterwards to update /boot/grub/grub.cfg.
+# For full documentation of the options in these files, see:
+#   info -f grub -n 'Simple configuration'
+
+GRUB_DEFAULT=0
+GRUB_TIMEOUT_STYLE=hidden
+GRUB_TIMEOUT=0
+GRUB_DISTRIBUTOR=`( . /etc/os-release && echo ${NAME} )`
+GRUB_CMDLINE_LINUX_DEFAULT="quiet splash amd_iommu=off"
+GRUB_CMDLINE_LINUX=""
+
+# If your computer has multiple operating systems installed, then you
+# probably want to run os-prober. However, if your computer is a host
+# for guest OSes installed via LVM or raw disk devices, running
+# os-prober can cause damage to those guest OSes as it mounts
+# filesystems to look for things.
+#GRUB_DISABLE_OS_PROBER=false
+
+# Uncomment to enable BadRAM filtering, modify to suit your needs
+# This works with Linux (no patch required) and with any kernel that obtains
+# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
+#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"
+
+# Uncomment to disable graphical terminal
+#GRUB_TERMINAL=console
+
+# The resolution used on graphical terminal
+# note that you can use only modes which your graphic card supports via VBE/GOP/UGA
+# you can see them in real GRUB with the command `videoinfo'
+#GRUB_GFXMODE=640x480
+
+# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
+#GRUB_DISABLE_LINUX_UUID=true
+
+# Uncomment to disable generation of recovery mode menu entries
+#GRUB_DISABLE_RECOVERY="true"
+
+# Uncomment to get a beep at grub start
+#GRUB_INIT_TUNE="480 440 1"