refactor: declare the toggle task fields once

[josegonzalez]

The three toggle tasks and dokku_maintenance each restated the same app and state field set, so every cross-cutting change had to be applied four times by hand. They now name a defined type over the shared ToggleFields. A defined type rather than an embedded struct keeps the field set flat to reflection, so the catalog, the required-field walk behind missing_required_field, the identity walk that names an unnamed task and the sensitive-value walks read the fields exactly as before. The parameter descriptions become plugin-agnostic as a result, which is what regenerates the task pages; each page's title and synopsis already name the plugin. TestToggleTasksDeclareTheSharedFields decides which tasks are toggles by whether their Plan() reaches planToggle rather than by their name, which is what keeps dokku_maintenance bound to the shared set despite not being spelled like one. The contributor docs described a planToggle signature removed in #322 and two envelope examples used a state: value no task accepts; both are corrected here. Fixes #467.