ci: gate the comment cap, which has been advisory since it was written

scripts/check-comment-runs.py was invoked by no workflow. The rule seven
agents spent a wave enforcing, and that carried seven silent successes of
its own, gated nothing.

Held back deliberately until every unit of the comment wave had landed: a
gate that passes before the work is done would be the same class of lie the
checker itself carried. Both checks pass against main as of 52507a6 —
"comment runs: none longer than 2 lines" and "451 entries, all name a test
that exists".

Closes improvise-9c8c.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -25,6 +25,12 @@ jobs:
       - run: cargo fmt --all -- --check
       - run: cargo clippy --workspace --all-targets -- -D warnings
       - run: cargo test --workspace
+      # The comment cap and the test-evidence keys (improvise-9c8c); without
+      # this step the rule is advisory. See context/design-principles.md §6.
+      - name: Comment runs capped at two lines
+        run: python3 scripts/check-comment-runs.py
+      - name: Test-evidence headings name real tests
+        run: python3 scripts/check-comment-runs.py --check-evidence
 
   # Too slow on GitHub Actions
   #