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/secrets.tf

@@ -75,12 +75,7 @@ locals {
       data          = { private_key = var.wireguard_private_key }
     }
 
-    # Nix cache signing keys
-    "hosts/nix-cache01/cache-secret" = {
-      auto_generate = false
-      data          = { key = var.cache_signing_key }
-    }
-
+    # Nix cache signing key
     "hosts/nix-cache02/cache-secret" = {
       auto_generate = false
       data          = { key = var.cache_signing_key_02 }