Fix nix-cache remote-builder trust: stale host key + wrong sshKey path

variables.nix's nixCacheHostKey no longer matched nix-cache's actual SSH
host key (confirmed via ssh-keyscan against the live container), so every
declaratively-configured client's programs.ssh.knownHosts trusted the
wrong key -- distributed builds would fail host-key verification. Also,
modules/nix-cache/remote-builder-client.nix hardcoded sshKey to
/root/.ssh/nixremote, but the `server` host only has its own default
/root/.ssh/id_ed25519 installed (confirmed live via qm guest-agent) --
that file was never even present, so the build machine config pointed at
nothing. Standardize on each client's own default identity, matching the
per-host-key pattern vars.remoteBuilderAuthorizedKeys already uses instead
of a shared/differently-named keypair, and add
scripts/secrets/sync-nix-cache-host-key.sh (wired into
codex-maintenance.sh's --check) so the host-key drift doesn't silently
recur next time nix-cache is rebuilt or recreated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V7yVH71vGrDVzovh9UaMu8

scripts/proxmox/configure-nix-cache-client.sh

@@ -15,6 +15,16 @@
 # this repo -- update both if nix-cache is ever rebuilt with a new host
 # key or the cache signing key is rotated (see docs/nix-cache.md).
 #
+# REMOTE_BUILDER_KEY defaults to this machine's own default root SSH
+# identity (matches modules/nix-cache/remote-builder-client.nix's
+# convention for real NixOS clients: authenticate as nixremote with the
+# host's own default key, added individually to
+# vars.remoteBuilderAuthorizedKeys, rather than a separately-named or
+# shared keypair) -- generate one with
+# `ssh-keygen -t ed25519 -N '' -f /root/.ssh/id_ed25519` if this machine
+# doesn't have one yet, then add its .pub to vars.remoteBuilderAuthorizedKeys
+# and rebuild nix-cache.
+#
 # Usage:
 #   sudo ./configure-nix-cache-client.sh [--dry-run] [--no-remote-builder] [--no-restart]
 #
@@ -24,9 +34,9 @@
 set -euo pipefail
 
 : "${NIX_CACHE_HOST:=nix-cache}"
-: "${NIX_CACHE_HOST_KEY:=ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHrMKZlIGUd3pH9G3AqbsruqUGjxIXMAZw52u9MwiBCn lxc-nix-cache}"
+: "${NIX_CACHE_HOST_KEY:=ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIPeWgMsdaiz4axT/deFc1+0B5bN+GX/NOeW9bbQ0c/IT lxc-nix-cache}"
 : "${REMOTE_BUILDER_USER:=nixremote}"
-: "${REMOTE_BUILDER_KEY:=/root/.ssh/nixremote}"
+: "${REMOTE_BUILDER_KEY:=/root/.ssh/id_ed25519}"
 
 CACHE_PUB_KEY="cache.local-1:usoWYanY3Kpq2+kDIS2nhWoLZiRxanmdysdzqCFBHW4="
 FALLBACK_URL="https://cache.nixos.org/"