fix(resume): capture error in handoff message fallback (#583)

When JSON parsing of inbox output fails, the code falls back to plain
text mode. However, the error from the fallback `gt mail inbox` command
was being silently ignored with `_`, masking failures and making
debugging difficult.

This change properly captures and returns the error if the fallback
command fails.

Co-authored-by: Gastown Bot <bot@gastown.ai>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

internal/cmd/resume.go

@@ -257,7 +257,10 @@ func checkHandoffMessages() error {
 	if err := json.Unmarshal(output, &messages); err != nil {
 		// JSON parse failed, use plain text output
 		inboxCmd = exec.Command("gt", "mail", "inbox")
-		output, _ = inboxCmd.Output()
+		output, err = inboxCmd.Output()
+		if err != nil {
+			return fmt.Errorf("fallback inbox check failed: %w", err)
+		}
 		outputStr := string(output)
 		if containsHandoff(outputStr) {
 			fmt.Printf("%s Found handoff message(s):\n\n", style.Bold.Render("🤝"))