feat(gateway): full Telegram+Discord integration with agent loop

Complete multi-platform gateway wiring:

config:
- [platform.telegram] section with bot_token
- [platform.discord] section with bot_token + guild_id
- Threaded through GyandootConfig → AgentConfig via typed structs

gateway:
- GatewayRunner::new(platforms) takes Vec<Box<dyn Platform>>
- start() spawns all platform pollers, merges receivers
- serve(agent) runs full message loop: receive → access control →
  typing indicator → session_turn() → platform.send()
- Ctrl+C graceful shutdown via tokio::select!

cmd_serve now:
1. Loads config, creates agent
2. Builds Telegram + Discord adapters from config
3. Starts gateway pollers
4. Enters serve loop until Ctrl+C

crates/gyandoot-core/src/lib.rs

@@ -11,11 +11,11 @@ pub mod service;
 
 pub use config::{
     AgentConfig, AgentSection, AnthropicConfig, BrowserSection, ConfigLoader,
-    GatewaySection, GyandootConfig, IdentitySection,
+    DiscordPlatformCfg, GatewaySection, GyandootConfig, IdentitySection,
     KanbanRootConfig, MemoryRootConfig, MemorySection, OllamaConfig,
     OpenAIConfig, PlatformRootConfig, PlatformSection, ProviderRootConfig,
     ProvidersSection, SecurityRootConfig, SkillsRootConfig, TelegramConfig,
-    WebhookConfig, DiscordConfig, load_config_from_file,
+    TelegramPlatformCfg, WebhookConfig, DiscordConfig, load_config_from_file,
 };
 pub use error::*;
 pub use types::*;