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/External/ExternalMcpService.cs

@@ -142,13 +142,18 @@ public sealed class ExternalMcpService
         return ToDto(task);
     }
 
-    [McpServerTool, Description("Create a new task in the given list. Set queueImmediately=true to enqueue it for agent execution.")]
+    [McpServerTool, Description(
+        "Create a new task in the given list. Set queueImmediately=true to enqueue it for agent execution. " +
+        "Set model to the cheapest model that can do the task well — 'haiku' for trivial/mechanical work, " +
+        "'sonnet' for normal coding (the default), 'opus' only for complex or cross-cutting work. " +
+        "Leave model null to inherit the list/global default.")]
     public async Task<TaskDto> AddTask(
         string listId,
         string title,
         string? description = null,
         string? createdBy = null,
         bool queueImmediately = false,
+        string? model = null,
         CancellationToken cancellationToken = default)
     {
         if (string.IsNullOrWhiteSpace(listId))
@@ -169,6 +174,7 @@ public sealed class ExternalMcpService
             CreatedAt = DateTime.UtcNow,
             CommitType = list.DefaultCommitType,
             CreatedBy = createdBy.NullIfBlank() ?? "mcp",
+            Model = ModelRegistry.NormalizeAlias(model),
         };
         await _tasks.AddAsync(entity, cancellationToken);