feat(cccl): integrate missing CCCL directories — python/, ci/, .agent/, docs/, test/

Sparse-checkout from NVIDIA/cccl main branch to complete cccl_upstream:

Added:
- python/cuda_cccl/ (226 files) — Python bindings for device-level algorithms
  Critical for muh toolchain: cuda.compute.reduce_into, scan, radix_sort, etc.
  Includes 204 .py files with full test coverage for all 27 algorithms
- ci/ (163 files) — Build/test infrastructure
  build_cub.sh, test_cub.sh, build_and_test_targets.sh, matrix.yaml
  Directly maps to our [INFRA-CI] and [INFRA-BUILD] items
- .agent/skills/ (7 files) — NVIDIA's own agent skills for CCCL
  cccl-style/SKILL.md, cccl-test/SKILL.md, sass-diff/SKILL.md
- docs/ (491 files) — Official CCCL documentation
  CI references, CMake guides, Python compute docs, libcudacxx PTX docs
- test/ (12 files) — Top-level integration tests (cuda_smoke, stdpar)
- Root configs: .clang-format, .clang-tidy, CONTRIBUTING.md, pyproject.toml
- CLAUDE.md symlink → AGENTS.md (NVIDIA's standard)

cccl_upstream now mirrors full NVIDIA/cccl structure:
  Before: 42M (cub + thrust + libcudacxx + cudax + c + examples + benchmarks)
  After:  53M (+python +ci +docs +.agent +test +configs)

This completes the CCCL base needed for:
- [muh-bench] items: ci/util/build_and_test_targets.sh for targeted builds
- [CCCL-verify] items: python/cuda_cccl/tests/ as reference implementations
- [CCCL-test] items: ci/test_cub.sh, ci/test_thrust.sh
- Agent workflow: .agent/skills/ for consistent style and test patterns

cccl_upstream/ci/test_packaging.sh

@@ -0,0 +1,71 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+ci_dir="$(dirname "${BASH_SOURCE[0]}")"
+cccl_dir="$(realpath "${ci_dir}/..")"
+
+MIN_CMAKE=false
+minimum_cmake_version=3.18.0
+
+new_args="$("${ci_dir}/util/extract_switches.sh" -min-cmake -- "$@")"
+declare -a new_args="(${new_args})"
+set -- "${new_args[@]}"
+while true; do
+  case "$1" in
+  -min-cmake)
+    MIN_CMAKE=true
+    shift
+    ;;
+  --)
+    shift
+    break
+    ;;
+  *)
+    echo "Unknown argument: $1"
+    exit 1
+    ;;
+  esac
+done
+
+if $MIN_CMAKE; then
+  echo "Installing minimum CMake version v${minimum_cmake_version}..."
+  wget -q \
+    https://github.com/Kitware/CMake/releases/download/v"${minimum_cmake_version}"/cmake-"${minimum_cmake_version}"-Linux-x86_64.sh \
+    -O /tmp/cmake-install.sh
+  prefix=/tmp/cmake-${minimum_cmake_version}
+  mkdir -p "${prefix}"
+  bash /tmp/cmake-install.sh --skip-license --prefix="${prefix}"
+  export MIN_CTEST_COMMAND="${prefix}/bin/ctest"
+fi
+
+# Needs to happen after cmake is installed:
+# shellcheck source=ci/build_common.sh
+source "${ci_dir}/build_common.sh"
+cd "${ci_dir}"
+
+print_environment_details
+
+PRESET="packaging"
+
+CMAKE_OPTIONS=()
+
+GPU_REQUIRED="true"
+
+CMAKE_OPTIONS=("-DCCCL_EXAMPLE_CPM_REPOSITORY=${cccl_dir}")
+
+# Local -- build against the current repo's HEAD commit:
+if [[ -z "${GITHUB_ACTIONS:-}" ]]; then
+  CMAKE_OPTIONS+=("-DCCCL_EXAMPLE_CPM_TAG=HEAD")
+else
+  CMAKE_OPTIONS+=("-DCCCL_EXAMPLE_CPM_TAG=${GITHUB_SHA:?}")
+fi
+
+if [[ -n "${MIN_CTEST_COMMAND:-}" ]]; then
+  CMAKE_OPTIONS+=("-DCCCL_EXAMPLE_CTEST_COMMAND=${MIN_CTEST_COMMAND}")
+fi
+
+configure_and_build_preset "Packaging" "$PRESET" "${CMAKE_OPTIONS[@]}"
+test_preset "Packaging" "$PRESET" "$GPU_REQUIRED"
+
+print_time_summary