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/helpers/common.bash

@@ -0,0 +1,30 @@
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+FIXTURES="$REPO_ROOT/tests/fixtures"
+MOCKS="$REPO_ROOT/tests/mocks"
+
+setup_mock_env() {
+    # Redirect ~/Applications/magick to our mock by overriding HOME
+    TEST_HOME="$(mktemp -d)"
+    mkdir -p "$TEST_HOME/Applications"
+    ln -s "$MOCKS/magick" "$TEST_HOME/Applications/magick"
+    export HOME="$TEST_HOME"
+
+    # Working dir for each test
+    TEST_WORKDIR="$(mktemp -d)"
+}
+
+teardown_mock_env() {
+    rm -rf "$TEST_HOME" "$TEST_WORKDIR"
+}
+
+make_zip() {
+    local zip_path="$1"; shift   # destination zip path
+    local -a jpegs=("$@")        # JPEG files to include
+    local tmpdir
+    tmpdir="$(mktemp -d)"
+    for jpg in "${jpegs[@]}"; do
+        cp "$jpg" "$tmpdir/"
+    done
+    (cd "$tmpdir" && zip -q "$zip_path" ./*.jpg)
+    rm -rf "$tmpdir"
+}