chore: import upstream snapshot with attribution

.github/actions/detect-changes/action.yml

@@ -0,0 +1,62 @@
+name: Detect affected areas
+description: >-
+  Classify a PR's changed files into CI work lanes (python, frontend, site,
+  scan, deps, mcp_catalog) so the orchestrator can conditionally call only
+  the sub-workflows a PR can affect. Outputs are always "true" on push/dispatch
+  events and fail open (everything "true") when the diff cannot be computed.
+
+outputs:
+  python:
+    description: Run Python tests / ruff / ty / windows-footguns.
+    value: ${{ steps.classify.outputs.python }}
+  frontend:
+    description: Run the TypeScript typecheck matrix + desktop build.
+    value: ${{ steps.classify.outputs.frontend }}
+  docker_meta:
+    description: Docker setup and meta files have changed.
+    value: ${{ steps.classify.outputs.docker_meta }}
+  site:
+    description: Build the Docusaurus docs site.
+    value: ${{ steps.classify.outputs.site }}
+  scan:
+    description: Run the supply-chain critical-pattern scanner.
+    value: ${{ steps.classify.outputs.scan }}
+  deps:
+    description: Check pyproject.toml dependency upper bounds.
+    value: ${{ steps.classify.outputs.deps }}
+  mcp_catalog:
+    description: Require MCP catalog security review label.
+    value: ${{ steps.classify.outputs.mcp_catalog }}
+
+runs:
+  using: composite
+  steps:
+    - name: Classify changed files
+      id: classify
+      shell: bash
+      env:
+        GH_TOKEN: ${{ github.token }}
+        REPO: ${{ github.repository }}
+        EVENT_NAME: ${{ github.event_name }}
+        BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+      run: |
+        set -euo pipefail
+
+        # Only pull_request events are gated. Other events (push, release,
+        # dispatch) leave CHANGED empty, so the classifier fails open and every
+        # lane runs. Post-merge / on-demand validation is never weakened.
+        if [ "$EVENT_NAME" = "pull_request" ]; then
+          # Use the compare endpoint with the pinned base/head SHAs from the
+          # event payload instead of the "current PR files" endpoint. The SHAs
+          # are frozen at trigger time, so the file list is deterministic even
+          # if the PR receives a new push between trigger and detect.
+          CHANGED="$(gh api \
+            --paginate \
+            "repos/${REPO}/compare/${BASE_SHA}...${HEAD_SHA}" \
+            --jq '.files[].filename' || true)"
+        fi
+
+        echo "Changed files:"
+        printf '%s\n' "${CHANGED:-(none)}"
+        printf '%s\n' "${CHANGED:-}" | python3 scripts/ci/classify_changes.py