docs: spec + plan for per-task model override via MCP

docs/superpowers/plans/2026-06-09-per-task-model-override-plan.md

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+# Plan: Per-task model override via MCP + cheapest-model prompt guidance
+
+Spec: `docs/superpowers/specs/2026-06-09-per-task-model-override-design.md`
+
+TDD, one focused commit per task. Build with `-c Release` per project; run
+`ClaudeDo.Worker.Tests` (and `Data.Tests` if touched).
+
+## Task 1 — ModelRegistry: cost ordering + alias validation
+
+- Add `ByCostAscending = ["haiku","sonnet","opus"]`.
+- Add `string? NormalizeAlias(string? model)`: trim; null/blank → null;
+  case-insensitive match against `Aliases` → canonical lowercase; else throw
+  `ArgumentException($"Unknown model '{model}'. Allowed: {join(Aliases)}.")`.
+- Tests (Data.Tests): "sonnet"/"OPUS"/" haiku " → normalized; ""/null/"  " →
+  null; "gpt4" → throws.
+
+## Task 2 — CreateChildAsync accepts model
+
+- `TaskRepository.CreateChildAsync`: add `string? model = null` (before the
+  trailing `CancellationToken ct = default`); set
+  `child.Model = ModelRegistry.NormalizeAlias(model)`.
+- Update the two existing callers to compile (named pass-through added in
+  Tasks 3–4; keep default null here).
+
+## Task 3 — Planning + improvement MCP tools forward model
+
+- `PlanningMcpService.CreateChildTask`: add `string? model` param after
+  `commitType`; pass to `CreateChildAsync`. Extend `[Description]` to document
+  the model arg (haiku/sonnet/opus; cheapest capable).
+- `TaskRunMcpService.SuggestImprovement`: add `string? model` param after
+  `description`; pass to `CreateChildAsync`. Extend `[Description]`.
+- Tests: each tool persists the model; invalid value throws.
+
+## Task 4 — External AddTask forwards model
+
+- `ExternalMcpService.AddTask`: add `string? model = null` param (before the
+  trailing `CancellationToken`); `entity.Model = ModelRegistry.NormalizeAlias(model)`.
+  Extend `[Description]`.
+- Test: AddTask persists model; invalid value rejected.
+
+## Task 5 — Prompt guidance
+
+- `PromptFiles.PlanningSystemDefault`: add a short paragraph — assign each
+  subtask the cheapest model that does it well, with ordering haiku < sonnet <
+  opus and the heuristic; pass it as `CreateChildTask(model=...)`.
+- `PromptFiles.SystemDefault` Out-of-scope section: when filing via
+  `SuggestImprovement`, pass the cheapest capable `model`.
+- `PromptFiles.ImprovementChildDefault`: one-line minimality reminder.
+- No test (static prompt text); verify build only.
+
+## Task 6 — Verify
+
+- Build App + Worker `-c Release`; run Worker.Tests + Data.Tests.
+- Update `ClaudeDo.Worker/CLAUDE.md` (ConfigMcpTools/creation-tool notes) and
+  `ClaudeDo.Data/CLAUDE.md` (ModelRegistry) if needed.