nix-cache02: add Harmonia binary cache service

- Parameterize harmonia.nix to use hostname-based Vault paths
- Add nix-cache services to nix-cache02
- Add Vault secret and variable for nix-cache02 signing key
- Add nix-cache02 public key to trusted-public-keys on all hosts
- Update plan doc to remove actions runner references

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

terraform/vault/secrets.tf

@@ -75,12 +75,17 @@ locals {
       data          = { private_key = var.wireguard_private_key }
     }
 
-    # Nix cache signing key
+    # Nix cache signing keys
     "hosts/nix-cache01/cache-secret" = {
       auto_generate = false
       data          = { key = var.cache_signing_key }
     }
 
+    "hosts/nix-cache02/cache-secret" = {
+      auto_generate = false
+      data          = { key = var.cache_signing_key_02 }
+    }
+
     # Homelab-deploy NKeys
     "shared/homelab-deploy/listener-nkey" = {
       auto_generate = false