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.Configuration/Extensions/ConfigurationBuilderExtensions.cs

@@ -34,6 +34,11 @@ public static class ConfigurationBuilderExtensions
                 builder.Add(new ProviderOptionsConfigurationSource(searchPath));
             }
 
+            // Re-applied last so environment variables take precedence over the TOML
+            // sources above. This is how secrets (API keys, bot tokens) are supplied —
+            // the embedded defaults ship with empty credentials.
+            builder.AddEnvironmentVariables();
+
             return builder;
         }
     }