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/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 = """