Correct the security claims in docs and comments, and record the accepted risks (closes #171)

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

internal/snapshot/scanner.go

@@ -119,7 +119,7 @@ type ScannerConfig struct {
 	Storage          storage.Storer
 	MaxBlobSize      int64
 	CompressionLevel int
-	AgeRecipients    []string   // Optional, empty means no encryption
+	AgeRecipients    []string   // required; output is always encrypted
 	EnableProgress   bool       // Enable the live progress reporter (ETAs, throughput)
 	UI               *ui.Writer // Where user-facing scanner messages go; nil = discard
 	Exclude          []string   // Glob patterns for files/directories to exclude