Add tag-driven GitHub release workflow with version gate

.github/workflows/release.yml

@@ -0,0 +1,48 @@
+name: Create release on tag
+
+on:
+  push:
+    tags:
+      - "v*.*.*"
+
+permissions:
+  contents: write
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Ensure tag commit is on main
+        run: |
+          git fetch origin main:refs/remotes/origin/main
+          git merge-base --is-ancestor "$GITHUB_SHA" "origin/main"
+
+      - name: Check version matches tag
+        run: |
+          TAG_VERSION="${GITHUB_REF_NAME#v}"
+          python - <<'PY'
+          import pathlib
+          import os
+
+          expected = os.environ["TAG_VERSION"]
+          values = {}
+          exec(pathlib.Path("version/version.py").read_text(), None, values)
+          actual = values["VERSION"]
+
+          if actual != expected:
+              raise SystemExit(
+                  f"version/version.py VERSION={actual!r} "
+                  f"does not match tag {expected!r}"
+              )
+          PY
+
+      - name: Create GitHub release
+        uses: softprops/action-gh-release@v2
+        with:
+          tag_name: ${{ github.ref_name }}
+          generate_release_notes: true

tests/test_release_workflow.py

@@ -0,0 +1,30 @@
+from pathlib import Path
+
+
+WORKFLOW_FILE = Path('.github/workflows/release.yml')
+
+
+def test_release_workflow_exists_and_triggers_on_version_tags():
+    assert WORKFLOW_FILE.exists()
+    content = WORKFLOW_FILE.read_text()
+    assert 'on:' in content
+    assert 'tags:' in content
+    assert '- "v*.*.*"' in content
+
+
+def test_release_workflow_checks_tag_commit_is_on_main():
+    content = WORKFLOW_FILE.read_text()
+    command = (
+        'git merge-base --is-ancestor "$GITHUB_SHA" '
+        '"origin/main"'
+    )
+    assert 'Ensure tag commit is on main' in content
+    assert command in content
+
+
+def test_release_workflow_checks_version_and_generates_notes():
+    content = WORKFLOW_FILE.read_text()
+    assert 'version/version.py' in content
+    assert 'VERSION' in content
+    assert 'softprops/action-gh-release@v2' in content
+    assert 'generate_release_notes: true' in content