feat(worker): build merge-helper interactive launch spec

src/ClaudeDo.Worker/Runner/Interfaces/IInteractiveLaunchSpecService.cs

@@ -26,4 +26,12 @@ public interface IInteractiveLaunchSpecService
     /// no task, no worktree, no session-skills seeding. Throws InvalidOperationException if the
     /// directory doesn't exist.</summary>
     Task<LaunchSpec> BuildForDirectoryAsync(string directory, CancellationToken ct);
+
+    /// <summary>Builds a LaunchSpec for an embedded ConPTY "merge helper" session that drives the
+    /// given tasks to a merged/Done state via the mcp__claudedo__* tools. Writes a per-session
+    /// system prompt + task brief under ~/.todo-app/merge-helper-sessions/&lt;guid&gt; and exposes
+    /// that dir plus every distinct existing repo dir via --add-dir. listId scopes the brief label
+    /// and the cwd to that list; null means all lists (cwd = first existing repo dir). Throws
+    /// InvalidOperationException if taskIds is empty or no task has an existing working directory.</summary>
+    Task<LaunchSpec> BuildForMergeHelperAsync(IReadOnlyList<string> taskIds, string? listId, CancellationToken ct);
 }