test: move bats scratch dirs to repo-local .test-scratch/ (per global no-/tmp/ rule)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests-impl/test-check-no-upstream-edits.bats

@@ -19,7 +19,8 @@ teardown() {
 # Build a minimal disposable repo that mimics the sethLabels structure with a
 # local "upstream/master" ref. Returns its path via stdout.
 make_test_repo() {
-  local tmp=$(mktemp -d)
+  mkdir -p "$REPO_ROOT/.test-scratch"
+  local tmp=$(mktemp -d -p "$REPO_ROOT/.test-scratch")
   cd "$tmp"
   git init -q -b master
   git config user.email "test@test"