fix: address review feedback — use helper wrappers, remove duplicates, fix scanStdin usage

- Replace bare fmt.Scanln with v.scanStdin() helper in snapshot.go
- Remove duplicate FetchBlob from vaultik.go (canonical version in blob_fetch_stub.go)
- Remove duplicate FetchAndDecryptBlob from restore.go (canonical version in blob_fetch_stub.go)
- Rebase onto main, resolve all conflicts
- All helper wrappers (printfStdout, printlnStdout, printfStderr, scanStdin) follow YAGNI
- No bare fmt.Print*/fmt.Scan* calls remain outside helpers
- make test passes: lint clean, all tests pass

internal/vaultik/snapshot.go

@@ -545,7 +545,7 @@ func (v *Vaultik) PurgeSnapshots(keepLatest bool, olderThan string, force bool)
 	if !force {
 		v.printfStdout("\nDelete %d snapshot(s)? [y/N] ", len(toDelete))
 		var confirm string
-		if _, err := fmt.Scanln(&confirm); err != nil {
+		if _, err := v.scanStdin(&confirm); err != nil {
 			// Treat EOF or error as "no"
 			v.printlnStdout("Cancelled")
 			return nil