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/libcudacxx/standard_api/concepts_library.rst

@@ -0,0 +1,49 @@
+.. _libcudacxx-standard-api-concepts:
+
+Concepts Library
+=======================
+
+.. list-table::
+   :widths: 25 45 30
+   :header-rows: 1
+
+   * - Header
+     - Content
+     - Availability
+   * - `\<cuda/std/concepts\> <https://en.cppreference.com/w/cpp/header/concepts>`_
+     - Fundamental library concepts
+     - CCCL 2.1.0 / CUDA 12.2
+
+Extensions
+----------
+
+-  All library features are available from C++14 onwards. The concepts
+   can be used like type traits prior to C++20.
+
+  .. code:: cpp
+
+    template<cuda::std::integral Integer>
+    void do_something_with_integers_in_cpp20(Integer&& i) {...}
+
+    template<class Integer, cuda::std::enable_if_t<cuda::std::integral<Integer>, int> = 0>
+    void do_something_with_integers_in_cpp17(Integer&& i) {...}
+
+    template<class Integer, cuda::std::enable_if_t<cuda::std::integral<Integer>, int> = 0>
+    void do_something_with_integers_in_cpp14(Integer&& i) {...}
+
+Restrictions
+------------
+
+-  Subsumption does not work prior to C++20
+
+  .. code:: cpp
+
+    template<class Integer, cuda::std::enable_if_t<subsuming_concept<Integer> && true, int> = 0>
+    void would_be_preferred_overload_in_cpp20(Integer&& i) {...}
+
+    template<class Integer, cuda::std::enable_if_t<cuda::std::integral<Integer>, int> = 0>
+    void is_always_ambiguous_in_cpp17(Integer&& i) {...}
+
+-  Subsumption is only partially implemented in the compiler until nvcc 12.4
+
+  nvcc has issues detecting subsumption of concepts that are composed of multiple concepts