plan should warn if target that has no default github runner is specified w/o a configured custom runner

we currently support an infinite number of targets in the targets array, but github has a finite and specified number of default runners. if someone adds a target that does not have a default runner, AND does not specify a custom runner, we should have plan error. ideally, this means that cargo-dist will catch the error on the configuration update and not when someone is trying to release.

Start by locating the plan validation entry point that processes the targets array and runner configuration. Trace how GitHub targets are checked, then add coverage showing that a target without a default GitHub runner or custom runner causes plan to fail before release; existing targets with either runner should remain valid.

Written by the indexing model from the issue text.