nix-cache01: decommission and remove all references

Removed:
- hosts/nix-cache01/ directory
- services/nix-cache/build-flakes.{nix,sh} (replaced by NATS builder)
- Vault secret and AppRole for nix-cache01
- Old signing key variable from terraform
- Old trusted public key from system/nix.nix

Updated:
- flake.nix: removed nixosConfiguration
- README.md: nix-cache01 -> nix-cache02
- Monitoring rules: removed build-flakes alerts, updated harmonia to nix-cache02
- Simplified proxy.nix (no longer needs hostname conditional)

nix-cache02 is now the sole binary cache host.

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

terraform/vault/approle.tf

@@ -87,13 +87,6 @@ locals {
       ]
     }
 
-    # Wave 5: nix-cache01
-    "nix-cache01" = {
-      paths = [
-        "secret/data/hosts/nix-cache01/*",
-      ]
-    }
-
     # vault01: Vault server itself (fetches secrets from itself)
     "vault01" = {
       paths = [