Add Luna.Configuration with TOML-based agent and provider config

Implement configuration system with embedded TOML resources for agent
and provider definitions, custom IConfigurationSource/Provider for
agent and provider options, and DI extension methods. Includes config
models for agents, providers, sessions, and Telegram integration.

Luna.Configuration/SessionOptions.cs

@@ -0,0 +1,7 @@
+namespace Luna.Configuration;
+
+public class SessionOptions
+{
+    public required float ContextTokenThreshold { get; init; }
+    public required int RetainedMessagesAfterCompacting { get; init; }
+}