Nothing routes to the explorer contexts after line actions and file entry
moved into the normal main pair. Delete their contexts, views, main-pair
wiring, discovery API, test drivers, and selection-state package so the
surviving diff view is the only implementation.
Their names go from the contexts a custom command may bind to as well, so
that a config still naming one is reported as the config error it now is
rather than taken as a context we simply failed to find.
Co-Authored-By: GitHub Copilot <copilot@github.com>