Hi!
With some help from bd808 and hashar, all extensions and skins now[1] have a "php-composer-validate" job, which will run "composer validate"[2] if your extension/skin's composer.json file is edited.
Some extensions are still not passing validation, so I've uploaded[3] some changes to make them all validate. Reviews appreciated :)
-- Legoktm
[1] https://gerrit.wikimedia.org/r/#/c/173457/ [2] https://getcomposer.org/doc/03-cli.md#validate [3] https://gerrit.wikimedia.org/r/#/q/status:open+topic:composer-validate,n,z