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();
+    }
+}