Add channel system with CLI, Telegram, and Web channel implementations

Define channel abstractions (IChannel, IChannelManager) with event-driven
message and connection handling. Implement ChannelManager for multi-channel
orchestration, CliChannel for terminal I/O, TelegramChannel with adapter
pattern for bot API integration, and WebChannel stub. Includes DI
registration and channel type enumeration.

Luna.Channels.Abstractions/IChannelManager.cs

@@ -0,0 +1,37 @@
+namespace Luna.Channels.Abstractions;
+
+/// <summary>
+/// Manages the lifecycle and routing of all registered communication channels.
+/// </summary>
+public interface IChannelManager
+{
+    /// <summary>
+    /// Registers a channel with this manager.
+    /// </summary>
+    /// <param name="channel"></param>
+    /// <exception cref="InvalidOperationException">Thrown if a channel with the same ID is already registered.</exception>
+    void RegisterChannel(IChannel channel);
+
+    /// <summary>
+    /// Unregisters a channel by its ID. No-op if the channel is not found.
+    /// </summary>
+    /// <param name="channelId">The ID of the channel to remove.</param>
+    void UnregisterChannel(string channelId);
+
+    /// <summary>
+    /// Retrieves a registered channel by its ID.
+    /// </summary>
+    /// <param name="channelId">The ID of the channel to retrieve.</param>
+    /// <returns>The channel, or null if not found.</returns>
+    IChannel? GetChannel(string channelId);
+
+    /// <summary>
+    /// Returns all currently registered channels.
+    /// </summary>
+    IReadOnlyList<IChannel> GetAllChannels();
+
+    /// <summary>
+    /// Raised after a message has been received and routed.
+    /// </summary>
+    event EventHandler<ChannelMessageReceivedEventArgs>? MessageRouted;
+}