Guard against empty stdout when resolving merge conflicts

Claude can exit 0 but produce no output, leaving the file blank.

src/commands/helpers.ts

@@ -127,7 +127,7 @@ export async function resolveConflicts(
       "resolve this merge conflict. output ONLY the resolved file content with no markdown fences, no explanation, no surrounding text.",
     )
 
-    if (resolved.exitCode !== 0) {
+    if (resolved.exitCode !== 0 || !resolved.stdout.trim()) {
       const stderr = resolved.stderr.trim()
       throw new Error(`Claude failed to resolve ${file}: ${stderr || "(no output)"}`)
     }