Working on designs for better [[chunking|design/assistant/chunks]]. Having
a hard time finding a way to totally obscure file sizes, but otherwise a
good design seems to be coming together. I particularly like that the new
design puts the chunk count in the Key (which is then encrypted for special
remotes, rather than having it be some special extension.

While thinking through chunking, I realized that the current chunking
method can fail if two repositories have different chunksize settings for
the same special remote and both upload the same key at the same time.
Arn't races fun? The new design will eliminate this problem; in the
meantime updated the docs to recommend never changing a remote's
chunksize setting.
