feat: rebuild base repository hierarchy, add readme and agents

- replace EfRepository/BaseDbContext/UtcValueConverter with BaseEfRepository and BaseRepository
- add IEfRepository interface hierarchy
- consolidate IEntity into Entity.cs, remove standalone IEntity.cs
- add PagedResult
- adjust all namespaces to HrynCo.DAL.Abstract / HrynCo.DAL.EF
- add README.md with solution overview, versioning rules, class diagram
- add AGENTS.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.DAL.Abstract/Entities/Entity.cs

@@ -1,9 +1,47 @@
 namespace HrynCo.DAL.Abstract.Entities;
 
-[Serializable]
-public abstract class Entity<TId> : IEntity<TId> where TId : struct
+public interface IEntity
 {
+    DateTimeOffset Created { get; set; }
+    object Id { get; set; }
+    DateTimeOffset? Updated { get; set; }
+}
+
+public interface IEntity<TId> : IEntity where TId : struct
+{
+    new TId Id { get; set; }
+}
+
+[Serializable]
+public class Entity<TId> : IEntity<TId> where TId : struct
+{
+    protected Entity()
+    {
+    }
+
+    public Entity(TId id)
+    {
+        Id = id;
+    }
+
     public TId Id { get; set; }
+
+    object IEntity.Id
+    {
+        get => Id;
+        set
+        {
+            if (value is TId typedValue)
+            {
+                Id = typedValue;
+            }
+            else
+            {
+                throw new InvalidCastException($"Cannot cast value of type {value.GetType()} to {typeof(TId)}.");
+            }
+        }
+    }
+
     public DateTimeOffset Created { get; set; }
     public DateTimeOffset? Updated { get; set; }
 }
@@ -17,7 +55,13 @@ public abstract class Entity : Entity<Guid>
     }
 
     protected Entity(Guid id)
+        : base(id)
     {
-        Id = id;
     }
 }
+
+[Serializable]
+public abstract class NamedEntity : Entity
+{
+    public required string Name { get; set; }
+}