fix: 增强 GPG 签名失败时的错误提示

src/git/commit.rs

@@ -174,8 +174,23 @@ impl CommitBuilder {
             .output()?;
         
         if !output.status.success() {
+            let stdout = String::from_utf8_lossy(&output.stdout);
             let stderr = String::from_utf8_lossy(&output.stderr);
-            bail!("Failed to amend commit: {}", stderr);
+            
+            let error_msg = if stderr.is_empty() {
+                if stdout.is_empty() {
+                    "GPG signing failed. Please check:\n\
+                     1. GPG signing key is configured (git config --get user.signingkey)\n\
+                     2. GPG agent is running\n\
+                     3. You can sign commits manually (try: git commit --amend -S)".to_string()
+                } else {
+                    stdout.to_string()
+                }
+            } else {
+                stderr.to_string()
+            };
+            
+            bail!("Failed to amend commit: {}", error_msg);
         }
         
         Ok(())