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/warp.rst

@@ -0,0 +1,57 @@
+.. _libcudacxx-extended-api-warp:
+
+Warp
+====
+
+.. toctree::
+   :hidden:
+   :maxdepth: 1
+
+   warp/warp_shuffle
+   warp/warp_match_all
+   warp/warp_match_any
+   warp/lane_mask
+
+.. list-table::
+   :widths: 25 45 30 30
+   :header-rows: 1
+
+   * - **Header**
+     - **Content**
+     - **CCCL Availability**
+     - **CUDA Toolkit Availability**
+
+   * - :ref:`warp_shuffle_idx <libcudacxx-extended-api-warp-warp-shuffle>`
+     - Warp shuffle from a specific lane
+     - CCCL 3.0.0
+     - CUDA 13.0
+
+   * - :ref:`warp_shuffle_up <libcudacxx-extended-api-warp-warp-shuffle>`
+     - Warp shuffle from original lane index - delta
+     - CCCL 3.0.0
+     - CUDA 13.0
+
+   * - :ref:`warp_shuffle_down <libcudacxx-extended-api-warp-warp-shuffle>`
+     - Warp shuffle from original lane index + delta
+     - CCCL 3.0.0
+     - CUDA 13.0
+
+   * - :ref:`warp_shuffle_xor <libcudacxx-extended-api-warp-warp-shuffle>`
+     - Warp shuffle from original lane index xor mask
+     - CCCL 3.0.0
+     - CUDA 13.0
+
+   * - :ref:`warp_match_all <libcudacxx-extended-api-warp-warp-match-all>`
+     - Check if all lanes have the same value
+     - CCCL 3.1.0
+     - CUDA 13.1
+
+   * - :ref:`lane_mask <libcudacxx-extended-api-warp-lane-mask>`
+     - Class to represent a mask of lanes in a warp
+     - CCCL 3.1.0
+     - CUDA 13.1
+
+   * - :ref:`warp_match_any <libcudacxx-extended-api-warp-warp-match-any>`
+     - Get the mask of lanes with the same value
+     - CCCL 3.5.0
+     - CUDA 13.5