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/mocks/magick

@@ -0,0 +1,24 @@
+#!/bin/bash
+# Mock for ~/Applications/magick.
+# identify: delegate to real system identify.
+# convert (resize call): copy in-place and embed the comment so the file
+#   looks processed on the next run.
+if [ "$1" = "identify" ]; then
+    shift
+    identify "$@"
+    exit $?
+fi
+
+# Parse -set comment <value> and the output file (last arg) from the argument list
+COMMENT=""
+OUTFILE="${@: -1}"
+args=("$@")
+for i in "${!args[@]}"; do
+    if [ "${args[$i]}" = "-set" ] && [ "${args[$i+1]}" = "comment" ]; then
+        COMMENT="${args[$i+2]}"
+    fi
+done
+
+if [ -n "$COMMENT" ]; then
+    convert "$OUTFILE" -set comment "$COMMENT" "$OUTFILE"
+fi