feat(data): session skills entity, repository, and migration

src/ClaudeDo.Data/Models/SessionSkillEntity.cs

@@ -0,0 +1,11 @@
+namespace ClaudeDo.Data.Models;
+
+public sealed class SessionSkillEntity
+{
+    public required string Name { get; init; }
+    public required string SourceUrl { get; set; }
+    public required string PinnedRef { get; set; }
+    public required string Subpath { get; set; }
+    public required string Description { get; set; }
+    public required DateTimeOffset AddedAt { get; set; }
+}