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>

.github/AGENTS.md

@@ -0,0 +1,30 @@
+# Guidelines for AI contributors
+
+These instructions help language models work with this repository.
+
+## Setup
+
+1. Run `npm install` to get linting tools and the Bats test framework.
+
+## Formatting
+
+- Format code and docs with Prettier and markdownlint:
+
+```bash
+npm run fix:prettier
+npm run fix:markdown
+```
+
+- Shell scripts should pass `shellcheck`.
+
+## Testing
+
+- When code changes, run `npm test` to execute Bats tests.
+- If only comments or documentation change, tests may be skipped.
+
+## Commits and PRs
+
+- Use Semantic Commit messages: `type(scope): summary`.
+- Keep PR titles in the same format.
+
+<!-- vim: set ft=markdown spell spelllang=en_us cc=80 : -->