Revert "Drop careless-gpu: the [cuda] pip extra install covers GPU and CPU nodes"

This reverts commit f6c2f5a650.

careless/README.md

@@ -11,10 +11,14 @@ inference (variational merging), built on TensorFlow.
 Upstream recommends a fresh conda env, but repo.anaconda.com is not
 reachable from Ra, so the build uses uv instead: it fetches a standalone
 Python 3.12 into `$PREFIX/python` (upstream supports >=3.9,<3.13), creates
-`$PREFIX/venv`, and runs `uv pip install careless[cuda]==<version>` into
-it. The `[cuda]` extra ships the NVIDIA libraries (CUDA, cuDNN, ...) as
-pip wheels, so the one install uses the GPU on GPU nodes (only the node's
-NVIDIA driver is needed — no cuda module) and falls back to CPU elsewhere.
+`$PREFIX/venv`, and runs `uv pip install careless==<version>` into it.
+This is the CPU build; the GPU build (with the `[cuda]` extra and the
+cluster `cuda/12.8.1` module) is the separate `careless-gpu` module.
+
+To share one uv download cache between the careless and careless-gpu
+builds, run each with `UV_CACHE_DIR=<shared path> modbuild build <version>`
+— a cache outside PREFIX is kept, an in-PREFIX cache is removed after a
+successful install.
 
 `uv` must be on PATH on the build host (e.g. `pip install --user uv`).