Restructure devcontainer: promote rust-only to root default, full stays as alternative

Co-authored-by: KSXGitHub <11488886+KSXGitHub@users.noreply.github.com>

Author: Copilot

.devcontainer/rust-only/devcontainer.json .devcontainer/devcontainer.json.devcontainer/rust-only/devcontainer.json renamed to .devcontainer/devcontainer.json

@@ -16,5 +16,5 @@
       ]
     }
   },
-  "postCreateCommand": "bash .devcontainer/rust-only/post-create.sh"
+  "postCreateCommand": "bash .devcontainer/post-create.sh"
 }