feat(worker): let Claude set the cheapest model per generated task via MCP

AddTask, planning CreateChildTask, and SuggestImprovement now accept an
optional alias-validated model (haiku/sonnet/opus; blank = inherit) so the
model is chosen at creation time instead of a follow-up set_task_config call.
The planning, system, and improvement prompts instruct Claude to pick the
cheapest capable model (haiku < sonnet < opus).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Worker/Runner/TaskRunMcpService.cs

@@ -25,10 +25,13 @@ public sealed class TaskRunMcpService
         "File an out-of-scope improvement as a child task of the current task. The child runs " +
         "automatically after this task finishes and is surfaced for review alongside it. Use ONLY " +
         "for work that is genuinely outside this task's scope (a refactor, follow-up, or tech debt) " +
-        "— never for work that belongs to the current task.")]
+        "— never for work that belongs to the current task. Set model to the cheapest model that can " +
+        "do the follow-up well — 'haiku' for trivial/mechanical work, 'sonnet' for normal coding, " +
+        "'opus' only for complex work. Leave model null to inherit the list/global default.")]
     public async Task<SuggestedImprovementDto> SuggestImprovement(
         string title,
         string description,
+        string? model,
         CancellationToken cancellationToken)
     {
         var callerId = _ctx.Current.CallerTaskId;
@@ -39,7 +42,7 @@ public sealed class TaskRunMcpService
                 "A child task cannot suggest further improvements (improvements are one layer deep).");
 
         var child = await _tasks.CreateChildAsync(
-            callerId, title, description, commitType: null, createdBy: callerId, cancellationToken);
+            callerId, title, description, commitType: null, createdBy: callerId, model: model, ct: cancellationToken);
         await _broadcaster.TaskUpdated(child.Id);
         await _broadcaster.TaskUpdated(callerId);
         return new SuggestedImprovementDto(child.Id);