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/docs/infrastructure/cmake/references/preset_reference.rst

@@ -0,0 +1,57 @@
+.. _infra-cmake-preset-reference:
+
+Preset reference
+================
+
+CCCL ships its CMake configurations as presets in ``CMakePresets.json``, using CMake's
+`preset format <https://cmake.org/cmake/help/latest/manual/cmake-presets.7.html>`_. The file
+defines 40+ presets covering each library, multiple C++ standards, special build
+modes, and benchmarking.
+
+Preset structure
+----------------
+
+Each preset belongs to one of three sections:
+
+- ``configurePresets`` set the configuration: enabled libraries, build type, CUDA
+  architectures, C++ standard, and per-library options. Every configure preset
+  inherits from the hidden ``base`` preset, which selects the Ninja generator,
+  Release mode, ``all-major-cccl`` architectures, and disables all libraries by
+  default. A named preset enables the libraries and options it needs.
+- ``buildPresets`` reference a configure preset by name. Some pin an explicit
+  target list; most build everything the configuration enables.
+- ``testPresets`` reference a configure preset and add CTest filters. Filters
+  select a subset of tests by name regex. CUB launcher-mode presets and Thrust
+  GPU/CPU splits are examples of this pattern.
+
+A configure preset, its build preset, and its test preset share a name. Run all
+three with the same ``<name>``.
+
+Listing available presets
+-------------------------
+
+``cmake --list-presets`` prints all configure presets::
+
+   cmake --list-presets
+
+List build and test presets separately::
+
+   cmake --list-presets=build
+   cmake --build --list-presets
+   ctest --list-presets
+
+Using a preset
+--------------
+
+Configure, then build with the matching preset name::
+
+   cmake --preset cub-cpp17
+   cmake --build --preset cub-cpp17
+
+Run the test preset of the same name with ``ctest``::
+
+   ctest --preset cub-cpp17
+
+Build output lands under ``build/<infix>/<presetName>/``, where the infix comes from
+the ``CCCL_BUILD_INFIX`` environment variable used for devcontainer isolation.
+Distinct presets use distinct build directories and do not collide.