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-test/SKILL.md

@@ -0,0 +1,13 @@
+---
+name: cccl-test
+description: Use when writing, updating, reviewing, or validating CCCL tests; read common CCCL test guidance and the path-specific references named by this skill.
+---
+
+# CCCL Test
+
+## Workflow
+
+1. Read `references/common.md` for guidance that applies across CCCL tests.
+2. For `libcudacxx/test/**/*`, also read `references/libcudacxx.md`.
+3. If no path-specific reference exists, follow nearby tests and repository docs. Do not import test rules from another subproject.
+4. Apply each reference only to its stated scope. Rules for one CCCL subproject do not automatically apply to another.

cccl_upstream/.agent/skills/cccl-test/references/common.md

@@ -0,0 +1,31 @@
+# Common CCCL Test Guidance
+
+Apply this guidance across CCCL tests unless a path-specific test reference says otherwise.
+
+## Local Consistency
+
+- Read nearby tests first and mirror their directory layout, file names, helper types, includes, assertion style, and local gating or skip mechanisms.
+
+## Coverage
+
+- Cover relevant edge cases.
+- Cover relevant input and output types.
+- Cover error behavior when applicable.
+- Cover runtime and compile-time behavior when applicable.
+- Cover device and host behavior when applicable.
+
+## Test Structure
+
+- All tests must have the correct license banner.
+- Use the local test harness assertions and helpers.
+- Use compile-time checks for compile-time guarantees and constexpr coverage when relevant.
+- Negative tests should check the intended diagnostic or failure mode when the local harness supports it.
+
+## Portability
+
+- Prefer project test macros and helpers for compiler, dialect, exception, host/device, and platform probes instead of spelling ad hoc checks directly.
+- If a test is unsupported, expected to fail, disabled, or skipped on a platform, motivate it with a comment.
+
+## Validation
+
+- Use targeted test runs for the project and files being changed.

cccl_upstream/.agent/skills/cccl-test/references/libcudacxx.md

@@ -0,0 +1,83 @@
+# libcudacxx Test
+
+## Organization
+
+- Put CUDA Standard Library tests under `libcudacxx/test/libcudacxx/std/...`.
+- Put CUDA-specific API tests under `libcudacxx/test/libcudacxx/cuda/...`, unless an adjacent `std/...` directory is clearly the established home for the functionality.
+- Read nearby tests first and mirror their directory layout, file names, helper types, includes, and lit gates.
+
+## Purpose
+
+- Validate libcudacxx functionality. It is fundamental to verify:
+
+  - Edge cases.
+  - Input and output types.
+  - Exception behavior.
+  - Runtime and constant-evaluation behavior.
+  - Device and host behavior.
+
+## Test kinds
+
+- `.pass.cpp`: compiles, links, runs, and returns 0.
+- `.compile.pass.cpp`: compiles correctly.
+- `.fail.cpp`: must fail compilation. Prefer precise `expected-error`, `expected-warning`, `expected-note`, or `expected-no-diagnostics` annotations when clang verify is supported.
+- `.runfail.cpp`: compiles and runs but must return non-zero.
+
+## Test structure
+
+- All tests must have the correct license banner.
+- Always include top level headers, never internal ones with `__` prefix.
+- Include support headers `"test_macros.h"`, `"test_iterators.h"`, `"test_comparisons.h"`, when needed.
+- Use `static_assert(...)` for compile-time guarantees and constexpr coverage.
+- Use `<cuda/std/cassert>` and `assert(...)` for runtime checks.
+- The `main` function must be present, dispatch runtime and static-evaluation tests, and return 0.
+
+## Style
+
+- Use `cuda::std` names, not `std::` names, unless the test is intentionally checking interoperability with host standard library types.
+- Do not fully qualify names in header includes unless the test is intentionally checking interoperability with host standard library types.
+- Mark helper functions that may run on host and device with `TEST_FUNC`; use `TEST_DEVICE_FUNC` for device-only helpers.
+  use `TEST_TILE_FUNC` for tile only helpers and `TEST_TILE_DEVICE_FUNC` for functions that can run on tile and device
+- `const`-qualification is discouraged.
+- Don't use `noexcept` for helper functions unless strictly necessary.
+- Do not use lambda expressions in host/device test code unless nearby tests already prove the pattern is supported.
+
+## Portability
+
+- Guard host-only or device-only behavior with `NV_IF_TARGET(NV_IS_HOST, (...))` and `NV_IF_TARGET(NV_IS_DEVICE, (...))` respectively.
+- Use `TEST_STD_VER`, `TEST_COMPILER`, `TEST_CUDA_COMPILER`, `TEST_HAS_EXCEPTIONS`, and `TEST_THROW` from `"test_macros.h"` instead of spelling compiler or dialect probes directly.
+- Unsupported platforms can be disabled with `UNSUPPORTED: <feature-name>` or `XFAIL: <feature-name>` lit directives. Some common feature names are `nvrtc`, `enable-tile`, `pre-sm-70`, `c++17`, `c++20`, `msvc`, `gcc-<version>`, or `clang-<version>`.
+
+  - Always motivate unsupported features with a comment.
+
+## Lit directives
+
+- Put lit directives near the top of the file before includes.
+- Common directives: `UNSUPPORTED:`, `XFAIL:`, `REQUIRES:`, `ADDITIONAL_COMPILE_DEFINITIONS:`, `ADDITIONAL_COMPILE_OPTIONS_HOST:`, `ADDITIONAL_COMPILE_OPTIONS_CUDA:`, `MODULES_DEFINES:`, and `CONSTEXPR_STEPS:`.
+- For diagnostics in `.fail.cpp`, annotate the exact line that should fail when possible:
+
+```cpp
+bad_expression(); // expected-error {{message fragment}}
+```
+
+- Prefer checking the intended diagnostic over accepting any compile failure.
+
+## Validation
+
+Use targeted libcudacxx lit runs. Paths passed to `--lit-precompile-tests` and `--lit-tests` are relative to `libcudacxx/test/libcudacxx/`.
+
+```bash
+ci/util/build_and_test_targets.sh \
+  --preset libcudacxx \
+  --lit-precompile-tests "std/algorithms/alg.nonmodifying/alg.any_of/any_of.pass.cpp" \
+  --lit-tests "std/algorithms/alg.nonmodifying/alg.any_of/any_of.pass.cpp"
+```
+
+If running lit directly, use the configured site file:
+
+```bash
+LIBCUDACXX_SITE_CONFIG=<path-to-cccl>/build/<preset>/libcudacxx/test/libcudacxx/lit.site.cfg \
+  lit -v libcudacxx/test/libcudacxx/<relative-test-path>
+```
+
+- Use `-Dexecutor=NoopExecutor()` for precompile-only validation when runtime execution is unavailable or GPU coverage is not required.