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/extended_api/thread_groups.rst

@@ -0,0 +1,64 @@
+.. _libcudacxx-extended-api-thread-groups:
+
+Thread Groups
+=============
+
+.. code:: cuda
+
+   struct ThreadGroup {
+     static constexpr cuda::thread_scope thread_scope;
+     Integral size() const;
+     Integral thread_rank() const;
+     void sync() const;
+   };
+
+The *ThreadGroup concept* defines the requirements of a type that represents a group of cooperating threads.
+
+The `CUDA Cooperative Groups Library <https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#group-collectives>`_
+provides a number of types that satisfy this concept.
+
+Data Members
+------------
+
+.. list-table::
+   :widths: 25 75
+   :header-rows: 0
+
+   * - ``thread_scope``
+     - The scope at which ``ThreadGroup::sync()`` synchronizes memory operations and thread execution.
+
+Member Functions
+----------------
+
+.. list-table::
+   :widths: 25 75
+   :header-rows: 0
+
+   * - ``size``
+     - Returns the number of participating threads.
+   * - ``thread_rank``
+     - Returns a unique value for each participating thread (``0 <= ThreadGroup::thread_rank() < ThreadGroup::size()``).
+   * - ``sync``
+     - Synchronizes the participating threads.
+
+Notes
+-----
+
+This concept is defined for documentation purposes but is not materialized in the library.
+
+Example
+-------
+
+.. code:: cuda
+
+   #include <cuda/atomic>
+   #include <cuda/std/cstddef>
+
+   struct single_thread_group {
+     static constexpr cuda::thread_scope thread_scope = cuda::thread_scope::thread_scope_thread;
+     cuda::std::size_t size() const { return 1; }
+     cuda::std::size_t thread_rank() const { return 0; }
+     void sync() const {}
+   };
+
+`See it on Godbolt <https://godbolt.org/z/6c16KxqY7>`_