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/variables.tf

@@ -40,16 +40,9 @@ variable "wireguard_private_key" {
   sensitive   = true
 }
 
-variable "cache_signing_key" {
-  description = "Nix binary cache signing key (nix-cache01)"
-  type        = string
-  sensitive   = true
-}
-
 variable "cache_signing_key_02" {
   description = "Nix binary cache signing key (nix-cache02)"
   type        = string
-  default     = "PLACEHOLDER"
   sensitive   = true
 }