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/ci/test_thrust.sh

@@ -0,0 +1,71 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+CPU_ONLY=false
+GPU_ONLY=false
+
+ci_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+
+new_args="$("${ci_dir}/util/extract_switches.sh" -cpu-only -gpu-only -- "$@")"
+declare -a new_args="(${new_args})"
+set -- "${new_args[@]}"
+while true; do
+  case "$1" in
+  -cpu-only)
+    ARTIFACT_TAG="test_cpu"
+    CPU_ONLY=true
+    shift
+    ;;
+  -gpu-only)
+    ARTIFACT_TAG="test_gpu"
+    GPU_ONLY=true
+    shift
+    ;;
+  --)
+    shift
+    break
+    ;;
+  *)
+    echo "Unknown argument: $1"
+    exit 1
+    ;;
+  esac
+done
+
+# shellcheck source=ci/build_common.sh
+source "${ci_dir}/build_common.sh"
+
+print_environment_details
+
+if [[ -n "${GITHUB_ACTIONS:-}" ]]; then
+  if ! $CPU_ONLY && ! $GPU_ONLY; then
+    echo "Error: test_thrust.sh requires -cpu-only or -gpu-only in CI" >&2
+    exit 1
+  fi
+  producer_id="$(util/workflow/get_producer_id.sh)"
+  run_command "📦  Unpacking test artifacts" \
+    "${ci_dir}/util/artifacts/download_packed.sh" \
+      "z_thrust-test-artifacts-${DEVCONTAINER_NAME:?}-$producer_id-$ARTIFACT_TAG" \
+      /home/coder/cccl/
+else
+  ./build_thrust.sh "$@"
+fi
+
+declare -a PRESET_GPU_PAIRS=()
+
+if $CPU_ONLY; then
+  PRESET_GPU_PAIRS+=("thrust-cpu:false")
+elif $GPU_ONLY; then
+  PRESET_GPU_PAIRS+=("thrust-gpu:true")
+else
+  PRESET_GPU_PAIRS+=("thrust-cpu:false" "thrust-gpu:true")
+fi
+
+for pair in "${PRESET_GPU_PAIRS[@]}"; do
+  PRESET="${pair%%:*}"
+  GPU_REQUIRED="${pair##*:}"
+  test_preset "Thrust (${PRESET})" "${PRESET}" "${GPU_REQUIRED}"
+done
+
+print_time_summary