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>

hosts/nix-cache02/default.nix

@@ -2,5 +2,6 @@
   imports = [
     ./configuration.nix
     ./builder.nix
+    ../../services/nix-cache
   ];
 }