feat(sops): add sops-nix input, core module and devShell tooling

Wires sops-nix into every machine via machines/core/sops.nix so hosts
decrypt with their existing SSH host key. Adds a devShell exposing
sops/age/ssh-to-age for the operator workflow, which also fixes .envrc's
previously no-op `use flake`.

machines/core/sops.nix

@@ -0,0 +1,31 @@
+# sops-nix plumbing — applied to every machine.
+# See docs/secrets.md for the day-to-day workflow.
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+{
+  # Each host defaults to its own secrets/<hostname>.yaml. Override per-secret
+  # with sops.secrets.<name>.sopsFile for shared files.
+  sops.defaultSopsFile = lib.mkDefault (../../secrets + "/${config.networking.hostName}.yaml");
+  sops.defaultSopsFormat = "yaml";
+  sops.validateSopsFiles = true;
+
+  # Decrypt with the host's own SSH host key, converted to age.
+  # Explicit rather than inheriting the openssh-derived default, so this does
+  # not silently break if services.openssh is ever disabled.
+  sops.age.sshKeyPaths = [ "/etc/ssh/ssh_host_ed25519_key" ];
+  sops.age.generateKey = false;
+
+  # Age only. The default here is the RSA host key imported as a GPG key,
+  # which is deprecated and an extra failure surface.
+  sops.gnupg.sshKeyPaths = [ ];
+
+  environment.systemPackages = with pkgs; [
+    sops
+    age
+    ssh-to-age
+  ];
+}