Step 25: Real FIDO2 hardware key support.

HardwareFIDO2 implements FIDO2Device via go-libfido2 (CGo bindings to
Yubico's libfido2). Gated behind //go:build fido2 tag to keep default
builds CGo-free. Nix flake adds sgard-fido2 package variant.

CLI changes: --fido2-pin flag, unlockDEK helper tries FIDO2 first,
add-fido2/encrypt init --fido2 use real hardware, auto-unlock added
to restore/checkpoint/diff for encrypted entries.

Tested manually: add-fido2, add --encrypt, restore, checkpoint, diff
all work with hardware FIDO2 key (touch-to-unlock, no passphrase).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd/sgard/restore.go

@@ -21,6 +21,12 @@ var restoreCmd = &cobra.Command{
 			return err
 		}
 
+		if g.HasEncryption() && g.NeedsDEK(g.List()) {
+			if err := unlockDEK(g); err != nil {
+				return err
+			}
+		}
+
 		confirm := func(path string) bool {
 			fmt.Printf("Overwrite %s? [y/N] ", path)
 			scanner := bufio.NewScanner(os.Stdin)