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>

README.md

@@ -12,7 +12,7 @@ NixOS Flake-based configuration repository for a homelab infrastructure. All hos
 | `http-proxy` | Reverse proxy |
 | `monitoring01` | Prometheus, Grafana, Loki, Tempo, Pyroscope |
 | `jelly01` | Jellyfin media server |
-| `nix-cache01` | Nix binary cache |
+| `nix-cache02` | Nix binary cache + NATS-based build service |
 | `nats1` | NATS messaging |
 | `vault01` | OpenBao (Vault) secrets management |
 | `template1`, `template2` | VM templates for cloning new hosts |