Register sops age key for proxmox-minimal

Adds the proxmox-minimal host's age key (derived from its SSH host
key via scripts/prepare-host-key.sh) as a recipient in .sops.yaml and
re-encrypts secrets/common.yaml for it via sops updatekeys.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

.sops.yaml

@@ -6,6 +6,7 @@ keys:
   - &lxc-minimal age1qz9d4ka4xgexujyd247s7lp737sulp5fhxl5d65fj2ykvc4j4edqrsdks8
   - &nix-minimal age120whqj96g26lsgy4udvgsn8dc9lumh8jeu3a564fx79rjr5lxffqmrljuu
   - &lxc-nix-cache age164px2a8e48ptsf9ngtan38aa6jls4jdl26mzrgzf6sn3vcvt49hqjrgr8w
+  - &proxmox-minimal age10at8862478urh0eeuwh8hzln6ck78jgwtztgxatwqlzwagg77y5snm4xzg
 
 creation_rules:
   # Shared across every currently-deployed host: root/nixos password hash,
@@ -22,6 +23,7 @@ creation_rules:
           - *lxc-minimal
           - *nix-minimal
           - *lxc-nix-cache
+          - *proxmox-minimal
 
   - path_regex: secrets/nix-cache\.yaml$
     key_groups: