feat(worker): render task descriptions into the list-handler brief

Phase 0 forced a batch_get_tasks full-fetch across every task just to see
descriptions, which blew past the client's token limit on larger lists.
brief.md lives on disk and has no such limit, so descriptions now render
there directly (fenced with an extended backtick run, indented under the
list bullet, so embedded headings/lists/code fences can't break the task
list's own structure). Phase 0 now treats the brief as the primary source
and only falls back to batch_get_tasks for fields it doesn't carry.

tests/ClaudeDo.Data.Tests/PromptFilesTests.cs

@@ -87,6 +87,15 @@ public class PromptFilesTests
         Assert.DoesNotContain("whenever you are unsure", d);
     }
 
+    [Fact]
+    public void DefaultFor_merge_helper_phase0_treats_brief_as_primary_source()
+    {
+        var d = PromptFiles.DefaultFor(PromptKind.MergeHelper);
+        Assert.Contains("primary source", d, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("batch_get_tasks", d);
+        Assert.Contains("Do not act on any single task before you have read", d);
+    }
+
     [Fact]
     public void DefaultFor_merge_helper_initial_has_repo_token()
     {