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/PagedResult.cs

@@ -0,0 +1,9 @@
+namespace HrynCo.DAL.Abstract;
+
+public sealed class PagedResult<T>
+{
+    public required IReadOnlyList<T> Items { get; init; }
+    public required int Page { get; init; }
+    public required int PageSize { get; init; }
+    public required int TotalCount { get; init; }
+}