feat(prompts): retry prompt from file, append only real captured errors

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

src/ClaudeDo.Worker/Runner/ClaudeProcess.cs

@@ -6,6 +6,8 @@ namespace ClaudeDo.Worker.Runner;
 
 public sealed class ClaudeProcess : IClaudeProcess
 {
+    public const string NoResultPrefix = "Claude exited with code";
+
     private readonly WorkerConfig _cfg;
     private readonly ILogger<ClaudeProcess> _logger;
 
@@ -100,7 +102,7 @@ public sealed class ClaudeProcess : IClaudeProcess
 
         var error = lastStderr.Length > 0
             ? lastStderr.ToString().Trim()
-            : $"Claude exited with code {exitCode} and no result.";
+            : $"{NoResultPrefix} {exitCode} and no result.";
 
         return new RunResult
         {