Refactor navigation command definition
Don't require trailing slash in prefix
Fix typo
Remove dumb comments
Undocument -default=both()

This was dumb.
Dogfood navcommand creation

Many of the built-in navigation commands have special behavior that
aren't currently accounted for in the publicly exposed interface.
That's fine.
Allow "collection" affinity

This is equivalent to "controller" for now, but allows us to add
additional nuance later, if necessary.