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/.agent/skills/cccl-style/references/libcudacxx.md

@@ -0,0 +1,55 @@
+# libcudacxx Style Guidance
+
+Use this reference for `libcudacxx/include/**/*` and `cudax/include/**/*`.
+
+## Naming Style
+
+All non-public symbols must be C++ reserved identifiers:
+
+- `_` for macros and template parameters, e.g. `_MY_MACRO`, `_MyParameter`.
+- `__` for all other symbols, e.g. `__my_variable`.
+- Never use reserved keywords, such as `__in`, `__out`, or `__inout` as variables, parameters, or function names.
+- Avoid single-letter template parameter names. Wrong: `_T`; correct: `_Tp`.
+
+## Class / Struct
+
+- Data member names have postfix `_`, e.g. `class __myclass { int __data_; };`.
+- Constructor parameter names should match class/struct data member names without the postfix `_`, e.g. `class __myclass { __myclass(int __data) : __data_(__data) {} };`.
+
+## Functions
+
+- Use `constexpr` for functions that do not depend on run-time features, such as pointers.
+- If the return type is not explicit (`auto`), then a trailing return type is strongly preferred.
+
+## Headers
+
+- Use the correct license:
+  - `libcudacxx/include/cuda/std` files ported from LLVM libc++ use the LLVM license.
+  - `libcudacxx/include/cuda/` files use Apache License v2.0 with LLVM Exceptions.
+- Headers use include guards with names derived from the uppercase full path and closing `#endif` comments repeating the guard name.
+- Right after the include guard, include:
+
+```cpp
+#include <cuda/std/detail/__config>
+
+#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
+#  pragma GCC system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
+#  pragma clang system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
+#  pragma system_header
+#endif // no system header
+```
+
+- The last included header before code must be `<cuda/std/__cccl/prologue.h>`, and `<cuda/std/__cccl/epilogue.h>` must appear at the end of the file.
+
+## Comments
+
+- Use Doxygen-style `//! @brief` comments.
+- Documented functions must include `//! @brief`, `//! @param[in/out/in,out]` for every parameter, and `//! @return` for non-void functions.
+- The `@brief/@param/@return` description must accurately reflect the current functionality of the function.
+
+## Compiler Compatibility
+
+- Do not use lambda expressions in device-only or host-device code.
+- Do not rely on deduction guides for initialization; use explicit template arguments instead.