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

Cargo.lock

@@ -734,8 +734,10 @@ dependencies = [
  "async-trait",
  "chrono",
  "futures",
+ "gyandoot-agent",
  "gyandoot-core",
  "gyandoot-memory",
+ "gyandoot-platform",
  "gyandoot-providers",
  "reqwest",
  "serde",