fix(worker): cap run-log read size and harden run-history tests

- GetTaskLog reads at most last 256 KB; prepends truncation marker if file exceeds cap
- Wrap temp-file cleanup in finally block to prevent leak on assertion failure
- Add GetRun_NotFound_Throws, GetTaskLog_RunExistsButNoLogPath_Throws, and GetTaskLog_LargeFile_ReturnsTruncatedTail tests

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Worker/External/RunHistoryMcpTools.cs

@@ -33,6 +33,8 @@ public sealed class RunHistoryMcpTools
         return ToDto(run);
     }
 
+    private const int MaxLogBytes = 256 * 1024;
+
     [McpServerTool, Description("Fetch the raw log output of a task's latest run. Throws if no log is available.")]
     public async Task<string> GetTaskLog(string taskId, CancellationToken cancellationToken)
     {
@@ -40,7 +42,17 @@ public sealed class RunHistoryMcpTools
             ?? throw new InvalidOperationException($"No runs found for task {taskId}.");
         if (string.IsNullOrWhiteSpace(run.LogPath) || !File.Exists(run.LogPath))
             throw new InvalidOperationException("No log available for the latest run.");
-        return await File.ReadAllTextAsync(run.LogPath, cancellationToken);
+
+        var totalBytes = new FileInfo(run.LogPath).Length;
+        if (totalBytes <= MaxLogBytes)
+            return await File.ReadAllTextAsync(run.LogPath, cancellationToken);
+
+        var buffer = new byte[MaxLogBytes];
+        await using var fs = new FileStream(run.LogPath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
+        fs.Seek(totalBytes - MaxLogBytes, SeekOrigin.Begin);
+        var read = await fs.ReadAsync(buffer, cancellationToken);
+        var tail = System.Text.Encoding.UTF8.GetString(buffer, 0, read);
+        return $"[truncated: showing last {MaxLogBytes} of {totalBytes} bytes]\n{tail}";
     }
 
     private static RunDto ToDto(TaskRunEntity r) => new(