Skip the candidate wheel build on PR heads that predate the release tooling

.github/workflows/pr.yaml

@@ -56,27 +56,49 @@ jobs:
         uses: actions/checkout@v6
         with:
           ref: ${{ github.event.pull_request.head.sha }}
+      - name: Detect the release tooling on the PR head
+        id: tooling
+        # The job builds from the reviewed head, not the merge commit, so a
+        # branch that predates the release tooling has no script to run.
+        # Skip the build with a notice instead of failing the PR; the
+        # ReleaseLock job above still checks the lock on the merge commit.
+        run: |
+          if [ -f .github/release_lock.py ] && [ -f .github/release_build.py ]; then
+            echo "present=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "present=false" >> "$GITHUB_OUTPUT"
+            echo "::notice title=Candidate wheel skipped::The PR head predates the release tooling (.github/release_lock.py and .github/release_build.py). Merge main into the branch to build its candidate wheel."
+          fi
       - name: Set up Python
+        if: steps.tooling.outputs.present == 'true'
         uses: actions/setup-python@v6
         with:
           python-version: "3.14.7"
       - name: Install uv
+        if: steps.tooling.outputs.present == 'true'
         uses: astral-sh/setup-uv@v8.1.0
         with:
           version: "0.12.13"
       - name: Check committed registry lock
+        if: steps.tooling.outputs.present == 'true'
         run: python .github/release_lock.py --committed
       - name: Predict release version with the release helper
+        if: steps.tooling.outputs.present == 'true'
         run: python .github/release_build.py --prepare "${{ matrix.kind }}"
       - name: Refresh only the release version in the registry lock
+        if: steps.tooling.outputs.present == 'true'
         run: python .github/release_lock.py --refresh
       - name: Install package
+        if: steps.tooling.outputs.present == 'true'
         run: uv sync --locked --extra dev
       - name: Build package from unchanged policy source
+        if: steps.tooling.outputs.present == 'true'
         run: uv run --no-sync make
       - name: Record and verify release build
+        if: steps.tooling.outputs.present == 'true'
         run: uv run --no-sync python .github/release_build.py
       - name: Retain unpublished wheel and build receipt
+        if: steps.tooling.outputs.present == 'true'
         uses: actions/upload-artifact@v4
         with:
           name: release-wheel-${{ matrix.kind }}-${{ github.event.pull_request.head.sha }}

changelog.d/ci-candidate-wheel-skip-legacy-heads.fixed.md

@@ -0,0 +1 @@
+Skip the pull-request candidate wheel build, with a notice, when the PR head predates the release tooling, instead of failing the check on every branch opened before that tooling landed.