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.Data.Tests/ModelRegistryTests.cs

@@ -0,0 +1,36 @@
+using ClaudeDo.Data.Models;
+
+namespace ClaudeDo.Data.Tests;
+
+public class ModelRegistryTests
+{
+    [Theory]
+    [InlineData("sonnet", "sonnet")]
+    [InlineData("OPUS", "opus")]
+    [InlineData(" haiku ", "haiku")]
+    public void NormalizeAlias_canonicalizes_known_aliases(string input, string expected)
+    {
+        Assert.Equal(expected, ModelRegistry.NormalizeAlias(input));
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    public void NormalizeAlias_blank_means_inherit(string? input)
+    {
+        Assert.Null(ModelRegistry.NormalizeAlias(input));
+    }
+
+    [Fact]
+    public void NormalizeAlias_unknown_throws()
+    {
+        Assert.Throws<ArgumentException>(() => ModelRegistry.NormalizeAlias("gpt4"));
+    }
+
+    [Fact]
+    public void ByCostAscending_is_haiku_sonnet_opus()
+    {
+        Assert.Equal(new[] { "haiku", "sonnet", "opus" }, ModelRegistry.ByCostAscending);
+    }
+}