userConfig never prompted the user at install time in testing, so all
credential-needing skills stay on .env.example + .env. Adds the missing
.env.example/.gitignore to check-my-timesheet and invoice-prep to match
invoice-checker and timesheet-checker.
Adds ivona (invoice-checker, timesheet-checker) and timesheet
(check-my-timesheet, invoice-prep) as new plugins, plus invoice-prep-test — a
one-off experiment that requests the Tempo API token via plugin userConfig at
install time instead of a .env file, to check whether that value actually
reaches the skill's script. Also adds the root LICENSE (MIT) referenced from
the README.
