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/IChannel.cs

@@ -0,0 +1,60 @@
+using Luna.Shared;
+
+namespace Luna.Channels.Abstractions;
+
+public delegate Task ChannelMessageReceivedEventHandler(object? sender, ChannelMessageReceivedEventArgs args);
+public delegate Task ChannelConnectionChangedEventHandler(object? sender, ChannelConnectionEventArgs args);
+
+/// <summary>
+/// Represents a communication channel for the Luna AI Assistant.
+/// Channels handle sending and receiving messages via different platforms
+/// (e.g., CLI, Telegram, Discord, Email).
+/// </summary>
+public interface IChannel : IDisposable
+{
+    /// <summary>
+    /// Unique identifier for this channel instance
+    /// </summary>
+    string ChannelId { get; }
+
+    /// <summary>
+    /// Type of channel (e.g., "cli", "telegram", "discord")
+    /// </summary>
+    string ChannelType { get; }
+
+    /// <summary>
+    /// Display name for the channel
+    /// </summary>
+    string DisplayName { get; }
+
+    /// <summary>
+    /// Indicates whether the channel is currently connected and ready
+    /// </summary>
+    bool IsConnected { get; }
+
+    /// <summary>
+    /// Event raised when a message is received from the channel
+    /// </summary>
+    event ChannelMessageReceivedEventHandler? MessageReceived;
+
+    /// <summary>
+    /// Event raised when the channel connection state changes
+    /// </summary>
+    event ChannelConnectionChangedEventHandler? ConnectionStateChanged;
+
+    /// <summary>
+    /// Send a message to the channel
+    /// </summary>
+    /// <param name="message">The message to send</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Task representing the send operation</returns>
+    Task SendMessageAsync(ChannelMessage message, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Send a streaming response to the channel
+    /// </summary>
+    /// <param name="messageStream">Async enumerable of response updates</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Task representing the send operation</returns>
+    Task SendStreamingMessageAsync(IAsyncEnumerable<ChatStreamUpdate> messageStream, CancellationToken cancellationToken = default);
+}