Release monthly from a scheduled workflow

The package version now comes from the latest v* git tag, so a release
is a tag and nothing else. A scheduled workflow tags main on the first
of the month when something was merged since the last release, creates
the GitHub release with generated notes, and publishes to PyPI and the
container registry. Pushing a v* tag by hand goes through the same path.

.github/workflows/release.yml

@@ -0,0 +1,44 @@
+name: Release
+
+on:
+  schedule:
+    - cron: "0 9 1 * *" # first of the month
+  workflow_dispatch:
+  push:
+    tags: ["v*"]
+
+jobs:
+  tag:
+    if: github.ref_type != 'tag'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      actions: write
+    env:
+      GH_TOKEN: ${{ github.token }}
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+      - run: |
+          last=$(git describe --tags --abbrev=0 --match 'v*')
+          if [ "$(git rev-list "$last..HEAD" --count)" = 0 ]; then
+            echo "Nothing merged since $last"; exit 0
+          fi
+          tag=v$(date -u +%Y.%-m.%-d)
+          gh release create "$tag" --target "$GITHUB_SHA" --generate-notes
+          # Events made with GITHUB_TOKEN don't start other workflows; run them by hand.
+          gh workflow run release.yml --ref "$tag"
+          gh workflow run publish-release-docker.yml --ref "$tag"
+
+  publish:
+    if: github.ref_type == 'tag'
+    runs-on: ubuntu-latest
+    permissions:
+      id-token: write # PyPI trusted publishing
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+      - uses: astral-sh/setup-uv@v10.0.0
+      - run: uv build && uv publish