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>

.gitignore

@@ -62,3 +62,6 @@ scripts/.cache/
 
 # Locally-produced AppImage artifacts
 *.AppImage
+
+# Bats test scratch directories (created/cleaned per test)
+.test-scratch/

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"

tests-impl/test-compute-version.bats

@@ -47,7 +47,8 @@ setup() {
 
 @test "fails cleanly when upstream/master ref is missing" {
   # Run in a temp git repo with no upstream remote.
-  tmp=$(mktemp -d)
+  mkdir -p "$REPO_ROOT/.test-scratch"
+  tmp=$(mktemp -d -p "$REPO_ROOT/.test-scratch")
   cd "$tmp"
   git init -q
   git commit --allow-empty -m "init" -q