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/docs/404_helper.inc.html

@@ -0,0 +1,63 @@
+<ul>
+    <div id="linkGen"></div>
+</ul>
+<script type="module">
+    import Fuse from 'https://cdn.jsdelivr.net/npm/fuse.js@7.0.0/dist/fuse.mjs'
+    // This page assumes that the user is already in a versioned subtree
+    // It is no longer the root 404 handler and is instead delagated to.
+    var localUri = window.location.pathname;
+    var hostUrl = window.location.protocol + "//" + window.location.host
+
+    let params = new URLSearchParams(document.location.search);
+
+    var rootPath = localUri.replace(/\/404_helper\.html/, "")
+
+    var rootUrl = hostUrl + rootPath
+    var pagelistUrl = rootUrl + "/pagelist.txt"
+
+    // // Branch is versioned if it leads with `unstable` or `vX.Y.Z`
+    // var isBranchVersioned = localUri.match(/^\/cccl\/(unstable|v\d+\.\d+\.\d+)/);
+
+    // // If the branch is not versioned, we redirect to a versioned branch keeping the URI.
+    // if (!isBranchVersioned) {
+    //     window.location.href = hostUrl + '/' + localUri.replace(/^cccl\//g, "cccl/unstable");
+    // }
+
+    var searchString = params.get("path");
+
+    if (searchString === "/" || searchString === null) {
+      window.location.href = rootUrl + "/index.html"
+    }
+
+    if (searchString !== null) {
+        fetch(pagelistUrl)
+            .then((response) => response.text())
+            .then((text) => {
+                const pagelist = text.split(",")
+
+                const options = {
+                  includeScore: true,
+                  threshold: 0.8,
+                  ignoreLocation: false
+                }
+
+                // cccl/device_vector.html => 'cccl 'device 'vector
+                console.log("query:" + searchString)
+                const fuse = new Fuse(pagelist, options)
+                // Include the ending token in the search to prioritize short matches
+                const result = fuse.search(searchString + "(end)", {limit: 10})
+
+                const listItem = '<li><a href=\"{2}{0}\">{0}</a> - Score: {1}</li>'.replace(/\{2\}/g, rootUrl)
+                document.getElementById('linkGen').innerHTML = ""
+                result.forEach(element => {
+                  const targetUri = element.item.replace("(end)", "")
+                  const targetScore = element.score.toFixed(2)
+                  document.getElementById('linkGen').innerHTML += listItem.replace(/\{0\}/g, targetUri).replace(/\{1\}/g, targetScore)
+                  // The match is near perfect, just link directly to it.
+                  if (targetScore <= 0.02 ) {
+                    window.location.href = rootUrl + targetUri
+                  }
+                });
+            })
+    }
+</script>