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-gateway/Cargo.toml

@@ -9,6 +9,8 @@ description = "GyanDoot v2 — Gateway for platform sessions, routing, streaming
 gyandoot-core = { path = "../gyandoot-core" }
 gyandoot-memory = { path = "../gyandoot-memory" }
 gyandoot-providers = { path = "../gyandoot-providers" }
+gyandoot-platform = { path = "../gyandoot-platform" }
+gyandoot-agent = { path = "../gyandoot-agent" }
 tokio.workspace = true
 serde.workspace = true
 serde_json.workspace = true