Shun node command

We want to be able to enforce that an instrumented node ignores one (or more) specified nodes when selecting the other parent. By configuring a super majority to shun nodes, they effectively go stale.

Start by clarifying the requested shun-node command and how parent selection should honor configured exclusions; the issue names no files or tests. Trace the existing command and parent-selection entry points, then define tests for ignored nodes and the super-majority stale-node behavior. Done when the behavior and acceptance tests are agreed and implemented.

Written by the indexing model from the issue text.