feat(worker): compose task prompt from title + description + open steps only

Resolved sub-tasks are no longer appended to the prompt. Extracted into a
shared TaskPromptComposer so the UI's description preview can render the same
'what Claude gets' text.

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

src/ClaudeDo.Worker/Runner/TaskRunner.cs

@@ -101,16 +101,10 @@ public sealed class TaskRunner
             await _state.StartRunningAsync(task.Id, now, ct);
             await _broadcaster.TaskStarted(slot, task.Id, now);
 
-            // Build prompt.
-            var sb = new System.Text.StringBuilder(task.Title);
-            if (!string.IsNullOrWhiteSpace(task.Description)) sb.Append("\n\n").Append(task.Description.Trim());
-            if (subtasks.Count > 0)
-            {
-                sb.Append("\n\n## Sub-Tasks\n");
-                foreach (var s in subtasks)
-                    sb.Append(s.Completed ? "- [x] " : "- [ ] ").Append(s.Title).Append('\n');
-            }
-            var prompt = sb.ToString();
+            // Build prompt: title + description + only the OPEN sub-tasks (resolved ones are dropped).
+            var prompt = TaskPromptComposer.Compose(
+                task.Title, task.Description,
+                subtasks.Select(s => (s.Title, s.Completed)));
 
             // Run 1.
             var result = await RunOnceAsync(task.Id, task.Title, slot, runDir, resolvedConfig, 1, false, prompt, ct);