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/util/create_mock_job_env.sh

@@ -0,0 +1,78 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+usage=$(cat <<EOF
+Usage: $0 <run id> <job_id>
+
+Allows the scripts in ci/util/workflow and ci/util/artifacts to run as though they are running in a CI environment.
+
+Create a mock job environment for testing purposes.
+
+The run id can be found in the workflow run URL, and the job_id can be found at the start of the Run Command job step.
+
+A new shell is spawned with the remote environment of a specific job from a specific workflow run.
+
+Environment variables are configured to mimic the CI environment.
+
+Caches and previously downloaded artifacts in /tmp are deleted to ensure a clean state.
+    !! Note that this does affect the caller's filesystem:
+       /tmp/workflow
+       /tmp/<artifact stages, archives, registry>
+       and similar caches will be deleted **from the caller's filesystem**.
+
+       This is usually fine, but be might overwrite files in-use by other mock environments.
+EOF
+)
+readonly usage
+
+if [[ "$#" -ne 2 ]]; then
+  echo "Error: Invalid number of arguments." >&2
+  echo "$usage" >&2
+  exit 1
+fi
+
+if [[ -n "${GITHUB_ACTIONS:-}" ]]; then
+  echo "$0: Detected another GITHUB_ACTIONS environment." >&2
+  echo "unset GITHUB_ACTIONS if this is intentional." >&2
+  exit 1
+fi
+
+if [[ -z "${DEVCONTAINER_NAME:-}" ]]; then
+  echo "This script must be run inside a devcontainer." >&2
+  exit 1
+else
+  echo "Running in devcontainer: $DEVCONTAINER_NAME"
+fi
+
+ci_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")/../" && pwd)
+
+export GITHUB_ACTIONS=true
+export GITHUB_RUN_ID="$1"
+export JOB_ID="$2"
+
+(
+  # shellcheck source=ci/util/workflow/common.sh
+  source "$ci_dir/util/workflow/common.sh"
+  # shellcheck source=ci/util/artifacts/common.sh
+  source "$ci_dir/util/artifacts/common.sh"
+
+  rm -rf "$WORKFLOW_DIR"
+  rm -rf "$ARTIFACT_ARCHIVES"
+  rm -rf "$ARTIFACT_UPLOAD_STAGE"
+  rm -rf "$ARTIFACT_UPLOAD_REGISTERY"
+)
+
+# Configure shell prompt:
+export PS0=""
+export PS1="<Mock Job: $GITHUB_RUN_ID $JOB_ID> [\u@\h \W]$ "
+export PROMPT_COMMAND=""
+
+
+echo "Starting new shell for emulating Job $JOB_ID in Run $GITHUB_RUN_ID".
+echo ""
+
+bash --norc --noprofile -i || :
+
+echo
+echo "Exiting mock job environment."