ARCHITECTURE.md: the chunk size limits, the commit date, the cases in
which a backup skips its prune, and two sentences that read badly.
docs/DATAMODEL.md: which repository type serves which table, what the
metadata export deletes and keeps, the blobs table in a restore, and
when a chunk row is deleted.
docs/REPOSTRUCTURE.md: how an object's size relates to its contents,
what blob sizes can leak and when, and a sentence moved out of the
prune rules.
All three now say that prune drops a local snapshot whose manifest is
gone, not whose directory is gone.
Model: fable-5-1
sneak
86c8d30def