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)}.");
+    }
 }

src/ClaudeDo.Data/PromptFiles.cs

@@ -82,7 +82,10 @@ public static class PromptFiles
         ## Out-of-scope improvements
         If you notice worthwhile work that is genuinely outside this task's scope
         (a refactor, a follow-up, tech debt), do NOT do it here. File it with
-        SuggestImprovement(title, description) and stay focused on the task at hand.
+        SuggestImprovement(title, description, model) and stay focused on the task at hand.
+        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 genuinely
+        complex work (cheapest to most capable: haiku < sonnet < opus).
 
         ## Working in the repo
         - Read a file before editing it. Match the conventions already in this codebase —
@@ -122,8 +125,8 @@ public static class PromptFiles
         # Out-of-scope follow-up
 
         You are an improvement follow-up that another task filed via SuggestImprovement.
-        It was deliberately scoped narrow. Do EXACTLY what this task's title and
-        description ask — nothing more.
+        It was deliberately scoped narrow, and is intentionally a small, cheap unit of
+        work. Do EXACTLY what this task's title and description ask — nothing more.
 
         - Make the smallest change that satisfies the task. No opportunistic refactors,
           renames, reformatting, or "while I'm here" cleanup beyond what is asked.
@@ -150,6 +153,14 @@ public static class PromptFiles
         Once the design is approved, create the child tasks with CreateChildTask, then
         call Finalize. Keep each subtask concrete and self-contained with a clear
         done-state, ordered so dependencies come first.
+
+        For each subtask, pass CreateChildTask's `model` argument set to the CHEAPEST
+        model that can do that subtask well. Models, cheapest to most capable:
+        haiku < sonnet < opus.
+        - haiku — trivial/mechanical work: doc tweaks, simple renames, small localized edits.
+        - sonnet — normal coding work; the sensible default when unsure.
+        - opus — only for genuinely complex, cross-cutting, or hard-to-debug work.
+        Do not default everything to opus — most subtasks are haiku or sonnet.
         """;
 
     private const string PlanningInitialDefault = """

src/ClaudeDo.Data/Repositories/TaskRepository.cs

@@ -197,6 +197,7 @@ public sealed class TaskRepository
         string? description,
         string? commitType,
         string? createdBy = null,
+        string? model = null,
         CancellationToken ct = default)
     {
         // AsNoTracking: SetPlanningStartedAsync mutates via ExecuteUpdate which
@@ -223,6 +224,7 @@ public sealed class TaskRepository
             ParentTaskId = parentId,
             SortOrder = (maxSort ?? -1) + 1,
             CreatedBy = createdBy,
+            Model = ModelRegistry.NormalizeAlias(model),
         };
         _context.Tasks.Add(child);
         await _context.SaveChangesAsync(ct);