terranix: convert to self-contained flake-parts modules

Each terranix configuration now exports its own flake output as a
self-contained module. Flattened directory structure and removed
centralized terranixConfigurations.nix in favor of import-tree.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

terranix/terminus.nix

@@ -0,0 +1,19 @@
+# OCI Terminus configuration placeholder
+{ inputs, ... }:
+
+{
+  imports = [ inputs.terranix.flakeModule ];
+
+  perSystem =
+    { pkgs, ... }:
+    {
+      terranix.terranixConfigurations.oci-terminus = {
+        terraformWrapper.package = pkgs.opentofu;
+        modules = [
+          ({ config, ... }: {
+            # Terraform config goes here
+          })
+        ];
+      };
+    };
+}