fix: the generated ruff config runs the rules it selects

.github/workflows/ci.yml

@@ -38,6 +38,35 @@ jobs:
           make test-unit
           uv build
 
+      # A rule that is selected and then ignored looks exactly like a rule that
+      # works: the gate above stays green either way. Hand the generated project
+      # the mistakes the selected rules exist to catch and make it say so, and an
+      # assert in tests/ that has to stay accepted.
+      - name: The generated config runs the rules it selects
+        working-directory: ${{ runner.temp }}/demo-lib
+        run: |
+          cat > demo_lib/probe.py <<'PY'
+          import datetime
+
+
+          def stamp(offset):
+              return datetime.datetime.now() + offset
+          PY
+          cat > tests/unit/test_probe.py <<'PY'
+          def test__assert__still_reads_as_a_test() -> None:
+              assert True
+          PY
+          report=$(uv run ruff check --output-format concise demo_lib/probe.py || true)
+          echo "$report"
+          for rule in DTZ005 ANN201 ANN001; do
+            case "$report" in
+              *"$rule"*) ;;
+              *) echo "::error::$rule never fired -- the config selects the rule and then ignores it"; exit 1 ;;
+            esac
+          done
+          uv run ruff check tests/unit/test_probe.py
+          rm demo_lib/probe.py tests/unit/test_probe.py
+
   all-checks-passed:
     name: All checks passed
     if: always()

template/CONTRIBUTING.md.jinja

@@ -26,6 +26,8 @@ make test             # full suite with 90% coverage threshold
 - **Docstrings** on public API only — Google style
 - **Line length** — 120 characters (ruff enforced)
 - **Quotes** — double quotes (ruff enforced)
+- **Timezone-aware datetimes** — `datetime.now(tz=...)`, never `utcnow()` (ruff `DTZ`)
+- **`assert` in tests only** — library code raises instead (ruff `S101`)
 - **No comments** unless the *why* is non-obvious
 
 ## Commit messages

template/pyproject.toml.jinja

@@ -86,8 +86,18 @@ target-version = "py{{ python_min_version | replace('.', '') }}"
 extend-exclude = ["*.md"]
 
 [tool.ruff.lint]
+# Everything selected here runs. An entry below names one rule the house style
+# disagrees with -- never a whole family that `select` has just asked for, which
+# reads as a promise the linter does not keep.
 select = ["F", "E", "W", "I", "B", "N", "S", "C4", "DTZ", "SIM", "TRY", "PERF", "RUF", "UP", "ANN", "T20", "PTH", "PLC", "PLE", "PLW"]
-ignore = ["TRY003", "S101", "ANN401", "RUF012", "S104", "S105", "S106", "ANN204", "DTZ", "ANN", "N802", "PERF401", "SIM105", "S607"]
+ignore = ["TRY003", "ANN401", "RUF012", "S104", "ANN204", "N802", "PERF401", "SIM105", "S607"]
+
+# Tests are held to a different standard from the library they exercise:
+# `assert` is what a test is made of, and a credential in a fixture is a literal
+# rather than a leak. Annotations are not on this list -- mypy reads
+# {{ package_name }}/ and never tests/, so ruff is the only thing holding that line here.
+[tool.ruff.lint.per-file-ignores]
+"tests/**/*.py" = ["S101", "S105", "S106"]
 
 [tool.ruff.format]
 quote-style = "double"