Correct remote layout and privacy docs for hashed snapshot keys (closes #67)

The remote layout and threat model in three documents described plaintext
snapshot IDs as directory names and misattributed the observable backup
time to those IDs. In fact `RemoteSnapshotKey` names each metadata
directory (and the manifest `snapshot_id`) with a one-way double SHA-256
hash of the human ID, so hostname and snapshot name are not observable;
the backup time is, via the plaintext manifest timestamp, an accepted
design property (issue 81).

Document the derivation once in `docs/REPOSTRUCTURE.md` with a worked
example; README, ARCHITECTURE and DATAMODEL now show the hashed layout and
link to it. Rewrite the privacy section to state what the unencrypted
manifest really exposes. Fix two code comments that claimed the public
bytes hide the timestamp. Docs and comments only; no behaviour change.

Model: opus-4-8

internal/snapshot/snapshot.go

@@ -840,8 +840,10 @@ func (sm *SnapshotManager) generateBlobManifest(
 	}
 
 	// Create manifest. SnapshotID in the unencrypted manifest is the
-	// double-SHA256 remote key, not the human ID, so the public bytes
-	// don't reveal hostname/snapshot-name/timestamp metadata.
+	// double-SHA256 remote key (see RemoteSnapshotKey), not the human ID,
+	// so neither this field nor the directory name reveals the hostname or
+	// snapshot name. Timestamp below is written in the clear, so the backup
+	// time is observable to anyone who can read the manifest.
 	manifest := &Manifest{
 		SnapshotID:          RemoteSnapshotKey(snapshotID),
 		Timestamp:           time.Now().UTC().Format(time.RFC3339),