Fix OpenCode backend invocation and diagnostics

src/backends/external.ts

@@ -37,6 +37,9 @@ function inferArgs(name: ExternalBackendName, prompt: string): string[] {
   if (name === 'claude_code') {
     return ['--print', prompt];
   }
+  if (name === 'opencode') {
+    return ['run', '--format', 'default', prompt];
+  }
   return ['-p', prompt];
 }
 
@@ -99,7 +102,11 @@ function execFileAsync(command: string, args: string[], timeoutMs: number): Prom
   return new Promise((resolve, reject) => {
     execFile(command, args, { timeout: timeoutMs, maxBuffer: 1024 * 1024 }, (error, stdout, stderr) => {
       if (error) {
-        reject(new Error(`${error.message}${stderr ? `\n${stderr}` : ''}`));
+        const details = [stderr, stdout]
+          .map((entry) => entry.trim())
+          .filter((entry) => entry.length > 0)
+          .join('\n');
+        reject(new Error(details ? `${error.message}\n${details}` : error.message));
         return;
       }
       resolve(stdout || '');