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>

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -729,6 +729,42 @@ public sealed class ExternalMcpServiceTests : IDisposable
         Assert.Equal(TaskStatus.Done, reloaded!.Status);
     }
 
+    // ── AddTask model override ────────────────────────────────────────────────
+
+    [Fact]
+    public async Task AddTask_NoModel_LeavesModelNull()
+    {
+        var listId = await SeedListAsync();
+        var sut = NewService();
+
+        var dto = await sut.AddTask(listId, "t", cancellationToken: CancellationToken.None);
+
+        var loaded = await _tasks.GetByIdAsync(dto.Id);
+        Assert.Null(loaded!.Model);
+    }
+
+    [Fact]
+    public async Task AddTask_PersistsNormalizedModel()
+    {
+        var listId = await SeedListAsync();
+        var sut = NewService();
+
+        var dto = await sut.AddTask(listId, "t", model: "HAIKU", cancellationToken: CancellationToken.None);
+
+        var loaded = await _tasks.GetByIdAsync(dto.Id);
+        Assert.Equal("haiku", loaded!.Model);
+    }
+
+    [Fact]
+    public async Task AddTask_RejectsUnknownModel()
+    {
+        var listId = await SeedListAsync();
+        var sut = NewService();
+
+        await Assert.ThrowsAsync<ArgumentException>(
+            () => sut.AddTask(listId, "t", model: "gpt4", cancellationToken: CancellationToken.None));
+    }
+
     // ── ContinueTask validation ───────────────────────────────────────────────
 
     [Fact]