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/python/cuda_cccl/tests/headers/test_cuda_cccl.py

@@ -0,0 +1,64 @@
+import pytest
+
+import cuda.cccl as cccl
+
+
+def test_version():
+    v = cccl.__version__
+    assert isinstance(v, str)
+
+
+@pytest.fixture
+def inc_paths():
+    return cccl.get_include_paths()
+
+
+def test_headers_has_cuda(inc_paths):
+    assert hasattr(inc_paths, "cuda")
+
+
+def test_headers_has_cub(inc_paths):
+    assert hasattr(inc_paths, "cub")
+
+
+def test_headers_has_cudacxx(inc_paths):
+    assert hasattr(inc_paths, "libcudacxx")
+
+
+def test_headers_has_thrust(inc_paths):
+    assert hasattr(inc_paths, "thrust")
+
+
+def test_headers_as_tuple(inc_paths):
+    tpl = inc_paths.as_tuple()
+    assert len(tpl) == 4
+
+    thrust_, cub_, cudacxx_, cuda_ = tpl
+    assert cuda_ == inc_paths.cuda
+    assert cub_ == inc_paths.cub
+    assert cudacxx_ == inc_paths.libcudacxx
+    assert thrust_ == inc_paths.thrust
+
+
+def test_cub_version(inc_paths):
+    cub_dir = inc_paths.cub / "cub"
+    cub_version = cub_dir / "version.cuh"
+    assert cub_version.exists()
+
+
+def test_thrust_version(inc_paths):
+    thrust_dir = inc_paths.thrust / "thrust"
+    thrust_version = thrust_dir / "version.h"
+    assert thrust_version.exists()
+
+
+def test_cudacxx_version(inc_paths):
+    cudacxx_dir = inc_paths.libcudacxx / "cuda"
+    cudacxx_version = cudacxx_dir / "version"
+    assert cudacxx_version.exists()
+
+
+def test_nv_target(inc_paths):
+    nv_dir = inc_paths.libcudacxx / "nv"
+    nv_target = nv_dir / "target"
+    assert nv_target.exists()