Remove hardcoded secrets; source credentials from configuration

The Telegram bot token was a const in AddChannels(), and the Mistral API
key was baked into the embedded Provider.Mistral.toml resource.

- Wire AddChannels() through the existing (previously unused)
  AddTelegramOptions(), which binds "Channels:Telegram" and validates on
  start. TelegramOptions is resolved from IOptions<> so TelegramAdapter
  and TelegramChannel are unchanged.
- Ship the embedded provider defaults with an empty ApiKey. Ollama's
  placeholder is now a non-secret literal.
- Re-apply AddEnvironmentVariables() last in AddLunaConfiguration() so
  env vars override the TOML sources; without this the embedded defaults
  would shadow injected credentials.

Credentials are now supplied via Providers__Mistral__ApiKey and
Channels__Telegram__BotToken, or via ~/.luna/providers/*.toml overrides.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Luna.Channels/Extensions/ServiceCollectionExtensions.cs

@@ -1,25 +1,27 @@
 using Luna.Channels.Abstractions;
 using Luna.Channels.Telegram;
 using Luna.Configuration;
+using Luna.Configuration.Extensions;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Options;
 using Telegram.Bot;
 
 namespace Luna.Channels.Extensions;
 
 public static class ServiceCollectionExtensions
 {
-    private const string TelegramBotToken = "REDACTED-TELEGRAM-BOT-TOKEN";
-
     extension(IServiceCollection services)
     {
         public IServiceCollection AddChannels()
         {
             services.AddSingleton<IChannelManager, ChannelManager>();
 
-            // Telegram
-            var telegramOptions = new TelegramOptions { BotToken = TelegramBotToken };
-            services.AddSingleton(telegramOptions);
-            services.AddSingleton<ITelegramBotClient>(_ => new TelegramBotClient(TelegramBotToken));
+            // Telegram — BotToken is bound from configuration ("Channels:Telegram"),
+            // supplied via the Channels__Telegram__BotToken environment variable.
+            services.AddTelegramOptions();
+            services.AddSingleton(provider => provider.GetRequiredService<IOptions<TelegramOptions>>().Value);
+            services.AddSingleton<ITelegramBotClient>(provider =>
+                new TelegramBotClient(provider.GetRequiredService<TelegramOptions>().BotToken));
             services.AddHostedService<TelegramAdapter>();
 
             return services;