Ci: Run the suite daily against the binder, and open an issue when it fails

.github/workflows/tests.yml

@@ -9,6 +9,10 @@ on:
   push:
     branches: ['work/**']
   workflow_dispatch:
+  # Daily against python-multiplatform develop as it is that day: the binder's contract moves under
+  # this package without a pull request here, and #41 found out only at the next PR.
+  schedule:
+    - cron: '17 3 * * *'
 
 permissions:
   contents: read
@@ -60,3 +64,27 @@ jobs:
       - run: pip install pytest mypy setuptools wheel
       - name: Run the suite
         run: python -m pytest tests -q -rs
+
+  # A failed daily run opens one issue (or comments on the open one) so a binder change that breaks
+  # this package is seen without anyone watching the Actions tab.
+  report-scheduled-failure:
+    needs: [no-binder, with-binder]
+    if: failure() && github.event_name == 'schedule'
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+    env:
+      GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      GH_REPO: ${{ github.repository }}
+      RUN_URL: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}
+    steps:
+      - name: Open or update the tracking issue
+        run: |
+          title="Daily tests fail against python-multiplatform develop"
+          number=$(gh issue list --state open --search "in:title \"$title\"" --json number --jq '.[0].number')
+          body="The scheduled run failed: $RUN_URL"
+          if [ -n "$number" ]; then
+            gh issue comment "$number" --body "$body"
+          else
+            gh issue create --title "$title" --body "$body"
+          fi

AGENTS.md

@@ -194,6 +194,8 @@ worktree so a test can see it. Read it in place.
 - The suite runs on GitHub Actions (`.github/workflows/tests.yml`) in both environments, without and
   with a python-multiplatform checkout, for every pull request into `develop`. Local runs are for
   quick checks of the part you changed; leave the full two-environment run to CI.
+  It also runs daily against python-multiplatform `develop` as it is that day; a failed daily run
+  opens (or comments on) one tracking issue.
 - `PythonMultiplatform` is read-only from this repository. Never write to it from here.
 - Record the before and after counts of every change; a change that only touches documentation
   must leave them identical.