fix(prompts): apply system default on every run; dedupe roadblocks

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

src/ClaudeDo.Worker/Runner/StreamAnalyzer.cs

@@ -86,7 +86,7 @@ public sealed class StreamAnalyzer
         TokensIn = _tokensIn,
         TokensOut = _tokensOut,
         ApiRetryCount = _apiRetryCount,
-        Blocks = _blocks,
+        Blocks = _blocks.Distinct().ToList(),
     };
 
     private string? FallbackResult()

src/ClaudeDo.Worker/Runner/TaskRunner.cs

@@ -380,7 +380,7 @@ public sealed class TaskRunner
             global = await settingsRepo.GetAsync(ct);
         }
 
-        var systemFile = PromptFiles.ReadOrNull(PromptKind.System);
+        var systemFile = PromptFiles.ReadOrDefault(PromptKind.System);
 
         var instructions = MergeInstructions(
             systemFile, global.DefaultClaudeInstructions, listConfig?.SystemPrompt, task.SystemPrompt);