feat: add bats tests, docs (#139)

* fix(test): ensure bats file list uses xargs

* docs(readme): use yarn for testing instructions

* fix(test): ensure pipelines fail properly

* docs(alias): fix table header

---------

Signed-off-by: Ismo Vuorinen <ismo@ivuorinen.net>

local/bin/x-thumbgen.md

@@ -0,0 +1,26 @@
+# x-thumbgen
+
+Generate thumbnails using ImageMagick (magick) with MIME type filtering.
+
+## Usage
+
+```bash
+x-thumbgen [options] source_directory
+```
+
+Options:
+
+- `-o DIR` – output directory (default: same as source)
+- `-s STR` – suffix for thumbnails
+- `-h` – show help
+
+Environment variables like `THUMB_BACKGROUND` control the background
+color.
+
+### Example
+
+```bash
+THUMB_BACKGROUND=black x-thumbgen -o ~/thumbs ~/images
+```
+
+<!-- vim: set ft=markdown spell spelllang=en_us cc=80 : -->