Enable focused tests as repository pre-commit hook

tests/changed_tests.bats

@@ -6,6 +6,9 @@ load helpers
 
 @test "changed-test wrapper is executable and uses staged diff by default" {
     [ -x "$REPO_ROOT/scripts/test-changed.sh" ]
+    [ -x "$REPO_ROOT/.githooks/pre-commit" ]
+    run grep -F 'scripts/test-changed.sh' "$REPO_ROOT/.githooks/pre-commit"
+    [ "$status" -eq 0 ]
     run grep -F 'git diff --cached' "$REPO_ROOT/scripts/test-changed.sh"
     [ "$status" -eq 0 ]
 }