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>

tests/ClaudeDo.Data.Tests/TaskPromptComposerTests.cs

@@ -0,0 +1,45 @@
+using ClaudeDo.Data;
+using Xunit;
+
+namespace ClaudeDo.Data.Tests;
+
+public class TaskPromptComposerTests
+{
+    [Fact]
+    public void Composes_title_description_and_open_steps()
+    {
+        var result = TaskPromptComposer.Compose(
+            "Refactor diff viewer",
+            "Share the row template.",
+            new (string, bool)[] { ("Done step", true), ("Open step", false) });
+
+        Assert.Equal(
+            "Refactor diff viewer\n\nShare the row template.\n\n## Sub-Tasks\n- [ ] Open step\n",
+            result);
+    }
+
+    [Fact]
+    public void Drops_resolved_steps_and_omits_section_when_none_open()
+    {
+        var result = TaskPromptComposer.Compose(
+            "Title",
+            "Desc",
+            new (string, bool)[] { ("a", true), ("b", true) });
+
+        Assert.Equal("Title\n\nDesc", result);
+    }
+
+    [Fact]
+    public void Omits_description_when_blank()
+    {
+        var result = TaskPromptComposer.Compose("Title", "   ", new (string, bool)[] { ("open", false) });
+
+        Assert.Equal("Title\n\n## Sub-Tasks\n- [ ] open\n", result);
+    }
+
+    [Fact]
+    public void Title_only_when_no_description_or_steps()
+    {
+        Assert.Equal("Just a title", TaskPromptComposer.Compose("Just a title", null, System.Array.Empty<(string, bool)>()));
+    }
+}