skills: point every test-editing path at tests/STYLE.md

/recipe-upgrade --with-tests, /ci-test-review and /cc-ci-tests-update all author
test changes, and all three had only 'never weaken a test' as guidance. That did
not stop the plausible failure: the fixture INSERTed rows into the app's database,
which was correct for v2 and silently wrong for v3, where a site must belong to a
team. Events were acked 202 and discarded; the recipe sat RED for six weeks.

The rule that would have prevented it — set state up through the app's own
interface, not its database — now lives in tests/STYLE.md in the cc-ci repo, and
each of the three paths is told to read it before editing a test.

.claude/skills/ci-test-review/SKILL.md

@@ -79,6 +79,10 @@ For each real (non-flaky) finding, write the actual fix and open a PR. **Never m
   it handles the mirror to `git.autonomic.zone/recipe-maintainers/<recipe>` (upstream
   `git.coopcloud.tech`). Keep the change **bounded** to the diagnosed root cause; don't rewrite the
   recipe.
+- **Before editing any test, read `tests/STYLE.md` in the cc-ci repo.** It encodes the rules a test
+  change must satisfy — set state up through the app's interface rather than its database, gate on
+  version instead of branching, correct the fixture/wait but NEVER the assertion, and diagnose from
+  the app's own telemetry before concluding a test is stale.
 - **CI-server-side fix → cc-ci PR.** Branch the cc-ci product repo
   (`recipe-maintainers/cc-ci`), apply the fix, and open the PR via the Gitea API (use the
   `GITEA_*` creds from `/srv/cc-ci/.testenv`). **Single-writer discipline:** work on a dedicated