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_python_common.sh

@@ -0,0 +1,30 @@
+set -euo pipefail
+
+function list_environment {
+  begin_group "⚙️ Existing site-packages"
+  pip freeze
+  end_group "⚙️ Existing site-packages"
+}
+
+function run_tests {
+  module=$1
+
+  pushd "../python/${module}" >/dev/null
+
+  TEMP_VENV_DIR="/tmp/${module}_venv"
+  rm -rf "${TEMP_VENV_DIR}"
+  python -m venv "${TEMP_VENV_DIR}"
+  # shellcheck disable=SC1091
+  . "${TEMP_VENV_DIR}/bin/activate"
+  echo 'cuda-cccl @ file:///home/coder/cccl/python/cuda_cccl' > /tmp/cuda-cccl_constraints.txt
+  run_command "⚙️  Pip install ${module}" pip install -c /tmp/cuda-cccl_constraints.txt ".[test]"
+  begin_group "⚙️ ${module} site-packages"
+  pip freeze
+  end_group "⚙️ ${module} site-packages"
+  run_command "🚀  Pytest ${module}" pytest -n "${PARALLEL_LEVEL:-$(nproc --all --ignore=1)}" -v ./tests
+  deactivate
+
+  popd >/dev/null
+
+  print_time_summary
+}