feat: add hardened base and per-edition image overlays

Add the files that land in the image, organised as layered rootfs trees
(base first, then the edition layer wins on conflict).

- base: hardened kernel cmdline + sysctl, default-deny nftables,
  key-only ssh, persistent journald, module blacklist, no core dumps,
  snapshot timer units, motd.
- server: Prometheus + auto-provisioned Grafana + Loki + promtail.
- workstation: dev profile and desktop sysctl relaxations (perf,
  rootless containers).
- cloud: cloud-init provisioning config.

overlays/server/etc/grafana/provisioning/dashboards/dashboards.yml

@@ -0,0 +1,13 @@
+# Arcline OS — Grafana dashboard provisioning (server edition)
+apiVersion: 1
+
+providers:
+  - name: Arcline
+    orgId: 1
+    folder: Arcline
+    type: file
+    disableDeletion: false
+    allowUiUpdates: true
+    updateIntervalSeconds: 30
+    options:
+      path: /etc/grafana/provisioning/dashboards