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/monitoring/blackbox.nix

@@ -21,7 +21,7 @@ let
     "https://pyroscope.home.2rjus.net"
     "https://pushgw.home.2rjus.net"
 
-    # Caddy auto-TLS on nix-cache01
+    # Caddy auto-TLS on nix-cache02
     "https://nix-cache.home.2rjus.net"
 
     # Caddy auto-TLS on grafana01

services/monitoring/prometheus.nix

@@ -178,9 +178,7 @@ in
           }
         ];
       }
-      # TODO: nix-cache_caddy can't be auto-generated because the cert is issued
-      # for nix-cache.home.2rjus.net (service CNAME), not nix-cache01 (hostname).
-      # Consider adding a target override to homelab.monitoring.scrapeTargets.
+      # Caddy metrics from nix-cache02 (serves nix-cache.home.2rjus.net)
       {
         job_name = "nix-cache_caddy";
         scheme = "https";

services/monitoring/rules.yml

@@ -171,37 +171,14 @@ groups:
           description: "NATS has {{ $value }} slow consumers on {{ $labels.instance }}."
   - name: nix_cache_rules
     rules:
-      - alert: build_flakes_service_not_active_recently
-        expr: count_over_time(node_systemd_unit_state{instance="nix-cache01.home.2rjus.net:9100", name="build-flakes.service", state="active"}[1h]) < 1
-        for: 0m
-        labels:
-          severity: critical
-        annotations:
-          summary: "The build-flakes service on {{ $labels.instance }} has not run recently"
-          description: "The build-flakes service on {{ $labels.instance }} has not run recently"
-      - alert: build_flakes_error
-        expr: build_flakes_error == 1
-        labels:
-          severity: warning
-        annotations:
-          summary: "The build-flakes job has failed for host {{ $labels.host }}."
-          description: "The build-flakes job has failed for host {{ $labels.host }}."
       - alert: harmonia_down
-        expr: node_systemd_unit_state {instance="nix-cache01.home.2rjus.net:9100", name = "harmonia.service", state = "active"} == 0
+        expr: node_systemd_unit_state{instance="nix-cache02.home.2rjus.net:9100", name="harmonia.service", state="active"} == 0
         for: 5m
         labels:
           severity: critical
         annotations:
           summary: "Harmonia not running on {{ $labels.instance }}"
           description: "Harmonia has been down on {{ $labels.instance }} more than 5 minutes."
-      - alert: low_disk_space_nix
-        expr: node_filesystem_free_bytes{instance="nix-cache01.home.2rjus.net:9100", mountpoint="/nix"} / node_filesystem_size_bytes{instance="nix-cache01.home.2rjus.net:9100", mountpoint="/nix"} * 100 < 10
-        for: 5m
-        labels:
-          severity: warning
-        annotations:
-          summary: "Disk space low on /nix for {{ $labels.instance }}"
-          description: "Disk space is low on /nix for host {{ $labels.instance }}. Please check."
   - name: home_assistant_rules
     rules:
       - alert: home_assistant_down