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.EF/Core/BaseRepository.cs

@@ -0,0 +1,25 @@
+namespace HrynCo.DAL.EF.Core;
+
+using System.Diagnostics.CodeAnalysis;
+using HrynCo.DAL.Abstract.Entities;
+using Microsoft.EntityFrameworkCore;
+
+[SuppressMessage("Major Code Smell", "S2436:Reduce the number of generic parameters",
+    Justification = "Generic design is intentional and improves reusability")]
+public abstract class BaseRepository<TEfRepository, TDbContext, TEntity, TEntityId>
+    where TEntity : class, IEntity<TEntityId>
+    where TDbContext : DbContext
+    where TEfRepository : BaseEfRepository<TDbContext, TEntity, TEntityId>
+    where TEntityId : struct
+{
+    private readonly Lazy<TEfRepository> _lazyEfRepository;
+
+    protected BaseRepository()
+    {
+        _lazyEfRepository = new Lazy<TEfRepository>(CreateEfRepository);
+    }
+
+    protected TEfRepository EfRepository => _lazyEfRepository.Value;
+
+    protected abstract TEfRepository CreateEfRepository();
+}