feat(amd): add opencode web server at opencode.amd.hoyer.world

Mirror of the sgx opencode setup: systemd service on port 4196 fronted
by nginx with a per-host ACME cert (DNS-01 via internetbs). Adds amd
key + path rule to .sops.yaml so secrets under .secrets/amd/ encrypt
for the host.

systems/x86_64-linux/amd/opencode.nix

@@ -0,0 +1,47 @@
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}:
+
+let
+  port = 4196;
+  user = "harald";
+  homeDir = "/home/harald";
+in
+{
+  systemd.services.opencode-serve = {
+    description = "OpenCode Web Server";
+    after = [ "network.target" ];
+    wantedBy = [ "multi-user.target" ];
+
+    environment = {
+      HOME = homeDir;
+    };
+
+    serviceConfig = {
+      Type = "simple";
+      User = user;
+      Group = "users";
+      WorkingDirectory = homeDir;
+      ExecStart = "${pkgs.opencode}/bin/opencode serve --hostname 127.0.0.1 --port ${toString port}";
+      Restart = "always";
+      RestartSec = 5;
+      EnvironmentFile = config.sops.secrets.opencode-web-password.path;
+
+      # Security hardening
+      PrivateTmp = true;
+      ProtectSystem = "strict";
+      ProtectHome = false;
+      NoNewPrivileges = true;
+      ReadWritePaths = [ homeDir ];
+    };
+  };
+
+  sops.secrets.opencode-web-password = {
+    sopsFile = ../../../.secrets/amd/opencode-web.yaml;
+    owner = user;
+    restartUnits = [ "opencode-serve.service" ];
+  };
+}