Add bats test suite for image processing scripts

Tests cover skip-already-processed logic, cleanup, idempotency, and
the config.sh single-source-of-truth invariant. bats-core, bats-assert,
and bats-support added as submodules under tests/libs/.

tests/config.bats

@@ -0,0 +1,27 @@
+#!/usr/bin/env bats
+
+load 'libs/bats-support/load'
+load 'libs/bats-assert/load'
+load 'helpers/common'
+
+@test "config.sh defines MAGICK_COMMENT" {
+    run bash -c "source '$REPO_ROOT/config.sh' && echo \"\$MAGICK_COMMENT\""
+    assert_success
+    assert_output "magick:resize=8mp,quality=85,sampling=4:2:0"
+}
+
+@test "config.sh defines MAX_PIXELS" {
+    run bash -c "source '$REPO_ROOT/config.sh' && echo \"\$MAX_PIXELS\""
+    assert_success
+    assert_output "8000000"
+}
+
+@test "MAGICK_COMMENT matches in images_resize.sh and zip_images_resize.sh" {
+    local comment_from_config comment_in_resize comment_in_zip
+    comment_from_config=$(bash -c "source '$REPO_ROOT/config.sh' && echo \"\$MAGICK_COMMENT\"")
+    comment_in_resize=$(grep 'MAGICK_COMMENT=' "$REPO_ROOT/images_resize.sh" | grep -v source || true)
+    comment_in_zip=$(grep 'MAGICK_COMMENT=' "$REPO_ROOT/zip_images_resize.sh" | grep -v source || true)
+    # Neither script should define MAGICK_COMMENT itself — only config.sh should
+    assert_equal "" "$comment_in_resize"
+    assert_equal "" "$comment_in_zip"
+}