Docs and comments only; no behaviour change. Corrects ten overclaims the
security review found: snapshot names are hashed but the hash uses no
secret, so a guessed hostname and name can be confirmed; a blob is named
by hex(SHA256(SHA256(uncompressed contents))), stated once in
docs/REPOSTRUCTURE.md and referenced elsewhere; double hashing does not
hide known content (blob packing does); age uses ChaCha20-Poly1305, not
XChaCha20; encryption is required, not optional; a snapshot is marked
complete before its metadata is uploaded; the export comment now matches
its only caller; deep verify detects corruption, not authorship; adding a
recipient does not reach existing data; restore examples target a
user-owned directory.
Adds an Accepted Risks subsection under Security Considerations with the
seven documented risks, cross-referenced from the README.
Model: opus-4-8
sneak