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/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.