fix: rewrite ServiceContainer resolution logic and add AGENTS.md

- Replace broken reflection-based GetService(Type, args) with
  direct ResolveService method
- Fix positional argument matching (type-based FirstOrDefault)
- Fix greedy constructor scoring (no double-count)
- Narrow TryGetService exception handling to InvalidOperationException
- Throw on duplicate registrations
- Add IDisposable support
- Add thread-safe singleton creation
- Create AGENTS.md with repo guide

AGENTS.md

@@ -0,0 +1,38 @@
+# Syrette — agent guide
+
+Minimal C# DI library targeting net8.0;net9.0;net10.0.
+
+## Build commands
+
+```bash
+dotnet build Syrette          # library only
+dotnet build                  # everything (includes DISandbox console app)
+dotnet pack Syrette -c Release --output .
+```
+
+SDK: .NET 10.0+ (see `global.json` — `rollForward: latestMinor`, `allowPrerelease: true`).
+
+## Project structure
+
+| Path | Purpose |
+|---|---|
+| `Syrette/` | Library (class lib). Entrypoints: `ServiceContainer.cs`, `ServiceDescriptor.cs`, `ServiceLifetime.cs` |
+| `DISandbox/` | Unofficial manual-test sandbox. Console app referencing `Syrette`. Not part of the shipped package. |
+
+No test project exists. CI has the test step commented out (`nuget-pkg-build.yml:24`).
+
+## NuGet publishing
+
+- CI (Gitea Actions) triggers on `v*` tags or `workflow_dispatch`.
+- Pushes to both nuget.org and a Gitea package feed.
+- Version is `0.0.1.8-alpha` per `Syrette.csproj`.
+
+## Library API
+
+- `ServiceContainer` exposes only `AddSingleton` / `AddTransient` (multiple overloads) and `GetService<T>()`, `GetService(Type, args)`, `TryGetService(Type, args)`, `GetServices<T>()`, `GetServiceTypes<T>()`.
+- All registration methods return `ServiceContainer` for fluent chaining.
+- Constructor selection is **greedy**: picks the constructor with the most parameters satisfiable by registered services or explicit args.
+- Only Singleton and Transient lifetimes; no scoped, no property/method injection.
+- `ServiceContainer : IDisposable` — disposes singleton instances that implement `IDisposable`.
+- Duplicate registrations (same `(ServiceType, ImplementationType)`) throw `InvalidOperationException`.
+- Resolution-time args (`GetService(Type, args)`) override registration-time args of the same type (type-based merge).