A (very) basic mockup screenshot how a context menu could look like for an invididual file can be found here: https://phabricator.kde.org/D17316 A context menu item in the project files tree would be nice, too. In this case, support for a context menu on folders there would be helpful; currently, only files can be right-clicked. See also https://phabricator.kde.org/D17449
This is a per-checker thing, not something we will support globally somehow. So, for e.g the "clazy" checker can check a single file even now. Other checkers that want to do this will need to support it individually.