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>

services/nix-cache/proxy.nix

@@ -1,14 +1,4 @@
-{ pkgs, config, ... }:
-let
-  # nix-cache02 serves the canonical nix-cache.home.2rjus.net
-  # nix-cache01 serves nix-cache01.home.2rjus.net (deprecated, pending decommission)
-  hostname = config.networking.hostName;
-  domain =
-    if hostname == "nix-cache02" then
-      "nix-cache.home.2rjus.net"
-    else
-      "${hostname}.home.2rjus.net";
-in
+{ pkgs, ... }:
 {
   services.caddy = {
     enable = true;
@@ -20,7 +10,7 @@ in
       }
 
 
-      ${domain} {
+      nix-cache.home.2rjus.net {
         log {
           output file /var/log/caddy/nix-cache.log {
             mode 644