Tech Debt: Rename prune/delete timeout to reconcile timeout

As a new user to this code base (through kpt & Config Sync), I was initially really confused about the difference between destroy, prune, and delete. And as a contributor, I have a hard time documenting/explaining the waiting behavior after deletion.

Solution:

• Rename prune to "delete". This will make the applier & destroyer more consistent and remove ambiguity between the two.
• Rename prune/delete timeout to "elimination timeout". This will parallel nicely with "reconciliation timeout", I think.

Both of these changes would be interface changes, for options and events, but I think the improved clarity will be worth it.

Destroy is also another synonym for delete, but in this context it means to delete a whole inventory, so we don't need to change that. I think it's easy to describe how destroy is different from delete/prune/elimination.

/cc @seans3 @mortent

The issue identifies prune/delete terminology, timeout names, options, and events as the affected surfaces, but names no files, tests, or entry points. Start by tracing those options and events through the Go repository, then confirm the intended rename scope and update coverage so the public terminology is consistent.