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.Data/TaskPromptComposer.cs

@@ -0,0 +1,29 @@
+using System.Text;
+
+namespace ClaudeDo.Data;
+
+/// <summary>
+/// Single source of truth for the text handed to Claude as a task prompt:
+/// title + description + the OPEN sub-tasks. Resolved sub-tasks are dropped.
+/// Shared by the Worker (real prompt) and the UI (the card's "what Claude gets" preview).
+/// </summary>
+public static class TaskPromptComposer
+{
+    public static string Compose(string title, string? description, IEnumerable<(string Title, bool Completed)> subtasks)
+    {
+        var sb = new StringBuilder((title ?? "").Trim());
+
+        if (!string.IsNullOrWhiteSpace(description))
+            sb.Append("\n\n").Append(description.Trim());
+
+        var open = subtasks?.Where(s => !s.Completed).ToList() ?? new List<(string, bool)>();
+        if (open.Count > 0)
+        {
+            sb.Append("\n\n## Sub-Tasks\n");
+            foreach (var s in open)
+                sb.Append("- [ ] ").Append(s.Title).Append('\n');
+        }
+
+        return sb.ToString();
+    }
+}

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);