feat(deploy flash): auto-install sops age key onto the SD boot partition

- after dd, if ~/.config/homelab/<config>/age.txt exists, mount the FAT boot
  partition and drop it as sops-age.txt (mercury). Key stays off-repo + out of
  the store + out of the image; no manual mount step.

hosts/mercury/secrets.nix

@@ -3,9 +3,9 @@
 # sops-nix wiring for mercury. Encrypted values in ../../secrets/mercury.yaml.
 #
 # SD images have no `--extra-files` step, so mercury uses a DEDICATED age key
-# (not ssh-host-key-derived) placed on the FAT boot partition after flashing:
-#   mount the boot partition, copy ~/.config/homelab/mercury/age.txt to it as
-#   sops-age.txt, unmount. The key never enters the repo or the nix store.
+# placed on the FAT boot partition. `./deploy flash mercury <dev>` does this
+# automatically (copies ~/.config/homelab/mercury/age.txt -> sops-age.txt).
+# The key never enters the repo, the nix store, or the image itself.
 {
   sops.defaultSopsFile = ../../secrets/mercury.yaml;
   sops.age.keyFile = "/boot/firmware/sops-age.txt";

scripts/deploy

@@ -11,7 +11,9 @@
 #   ./deploy boot    <config> <host>  stage for next boot, don't activate now.
 #   ./deploy test    <config> <host>  activate without adding a boot entry.
 #   ./deploy image   <config>         build an SD-card image (e.g. rpi mercury).
-#   ./deploy flash   <config> <dev>   build the SD image + write it to <dev>.
+#   ./deploy flash   <config> <dev>   build SD image, write to <dev>, and (if
+#                                     ~/.config/homelab/<config>/age.txt exists)
+#                                     drop the sops key on its boot partition.
 #
 # <config> = a nixosConfigurations name (e.g. jupiter, vps). Its pre-generated
 # SSH host key must be at ~/.config/homelab/<config>/ssh_host_ed25519_key.
@@ -115,6 +117,24 @@ case "$cmd" in
     [ "$ok" = yes ] || die "aborted"
     zstdcat "$img" | sudo dd of="$dev" bs=4M status=progress oflag=sync
     sync
+
+    # If this config has a dedicated sops age key, drop it on the FAT boot
+    # partition (mounts at /boot/firmware) so sops decrypts on first boot.
+    # Key stays off-repo and out of the nix store; the image itself has no key.
+    keyfile="$HOME/.config/homelab/$config/age.txt"
+    if [ -f "$keyfile" ]; then
+      echo ">> installing sops age key onto the boot partition"
+      sudo partprobe "$dev" 2>/dev/null || sudo blockdev --rereadpt "$dev" 2>/dev/null || true
+      sudo udevadm settle 2>/dev/null || true
+      fatpart="$(lsblk -lno PATH,FSTYPE "$dev" | awk '$2=="vfat"{print $1; exit}')"
+      [ -n "$fatpart" ] || die "no FAT boot partition found on $dev — copy $keyfile to it manually as sops-age.txt"
+      mnt="$(mktemp -d)"
+      sudo mount "$fatpart" "$mnt"
+      sudo cp "$keyfile" "$mnt/sops-age.txt"
+      sudo sync
+      sudo umount "$mnt"; rmdir "$mnt"
+      echo ">> age key installed (sops-age.txt)"
+    fi
     echo ">> done — insert the card into the Pi and boot."
     ;;
 

secrets/mercury.yaml

@@ -1,4 +1,4 @@
-darman_password: ENC[AES256_GCM,data:HzFkwGIyQL1sbPegJq970n3VASkaO/iY1t3fjodqk6FSoGX+4zlVoZfNAuTeiyAwv+0bZHhJoK+cAPKp5HMqsnhXJrhJUV7KVA3beyBXPPg8IpXi4X4maKv9KLIiWi2BJQuE421LUjb/2A==,iv:dcCD3aaEfVfPBh0jwFZkmNz0L/ZfRzMROJIHXd6pkG8=,tag:5vRhPo+fE28HrZPp6ZJ1MQ==,type:str]
+darman_password: ENC[AES256_GCM,data:vlcmADPCvt3H+etrXeowGl8w5+RQj6vwPzrx0+3qXb2U7s22H7VN8oXSwnMHQh+lAacrnl1c8u4D3SOh22QQBIix1wgAMQTFN15fo1RE3bTAUtiaV4HKfPNEsM+brTQs4oaX2Pnl3kZAbQ==,iv:DSTreGrypTYm0fAf8k6YZ277XMKYrocwPUUG0IBs4Jg=,tag:GuujksyB01YuX3rp82Jz3Q==,type:str]
 sops:
     age:
         - enc: |
@@ -19,7 +19,7 @@ sops:
             ZsBrZS0j0B3MhTI+6gUWB82xozgmnh95w+SXSSZZvoss8RuicE5ysA==
             -----END AGE ENCRYPTED FILE-----
           recipient: age1cpty7zrgnn6l97upq00w5wa8zcvnkxkdt2jvhlj97jh83exure4slha43t
-    lastmodified: "2026-07-13T18:50:12Z"
-    mac: ENC[AES256_GCM,data:8e1VHvwPzI97toTrkbT53ky8OV8Jepyn5yhEHq87wcDP4cywZTm6WbJsqJNgvYdwwxIgaLK7WyausaFfnqIBRFSxauhmTD7mjXKwGFAvSyeyFL3RYbdTc3+GInTE8gF8oBQ7OGIREW6pcDFswzhmcTKbJ01V+vJ2sgxnvifNe7E=,iv:pmLsvkPro4aw//UdldKErbYfVmlK+xb2/4CbN0I6gQQ=,tag:sy9wBkTYIdYrlJ8U4TdJIw==,type:str]
+    lastmodified: "2026-07-13T18:55:42Z"
+    mac: ENC[AES256_GCM,data:dVDVsrqz10/EhNCqVRRTKo8tKNCBTU8ZLA7ASE7exWzzUVB1LPIw4ht4QV88cd7iMKy10UMnLUjyg7m0RTbTEJXzlMHjDUbJThNCZ+/oQyQbGlwvCV61KIwFWK3G1bO3Bz6ux+Lo7IG3a8pJIjntTFfb4+l1612QRd73XHp8wT4=,iv:8kyta/3+wXRnC05GmNbDz6IA/AEFSZl1VM4QF6C0Nxc=,tag:rQf5GUelRFZVmdXK4DjcBw==,type:str]
     unencrypted_suffix: _unencrypted
     version: 3.13.1