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.Data/Models/ModelRegistry.cs

@@ -4,9 +4,26 @@ public static class ModelRegistry
 {
     public static readonly IReadOnlyList<string> Aliases = new[] { "sonnet", "opus", "haiku" };
 
+    /// <summary>Model aliases ordered cheapest → most capable. Single source for prompt cost guidance.</summary>
+    public static readonly IReadOnlyList<string> ByCostAscending = new[] { "haiku", "sonnet", "opus" };
+
     public const string DefaultAlias = "sonnet";
     public const string PlanningAlias = "opus";
 
     public const string ListDefaultSentinel = "(default)";
     public const string TaskInheritSentinel = "(inherit)";
+
+    /// <summary>
+    /// Validate a model alias from external input. Null/blank → null (inherit).
+    /// Returns the canonical lowercase alias; throws on an unknown value.
+    /// </summary>
+    public static string? NormalizeAlias(string? model)
+    {
+        var m = model?.Trim();
+        if (string.IsNullOrEmpty(m)) return null;
+        foreach (var alias in Aliases)
+            if (string.Equals(alias, m, StringComparison.OrdinalIgnoreCase))
+                return alias;
+        throw new ArgumentException($"Unknown model '{model}'. Allowed: {string.Join(", ", Aliases)}.");
+    }
 }