fix: handle error return from identity.Sign() in claim command

The core identity.Sign() signature changed from returning a single
string to returning (string, error). Update the claim command to
propagate the error with an appropriate message and exit code.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gcmsg

internal/cmd/claim.go

@@ -48,7 +48,11 @@ func runAgentClaim(args []string, serverURL string) int {
 	}
 
 	// Sign the token to prove key ownership.
-	sig := identity.Sign(kp.PrivateKey, []byte(*token))
+	sig, err := identity.Sign(kp.PrivateKey, []byte(*token))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error signing token: %v\n", err)
+		return 1
+	}
 
 	var caps []string
 	if *capabilities != "" {