feat: add channel adapter abstraction with Telegram and WebChat adapters

Implement Phase 3 channel adapters that decouple message sources from
the agent via a uniform ChannelAdapter interface and ChannelRegistry.

- Add ChannelAdapter/InboundMessage/OutboundMessage types
- Add ChannelRegistry for adapter lifecycle and message routing
- Add TelegramAdapter (grammy bot, auth middleware, confirmations, chunking)
- Add WebChatAdapter (thin shim over GatewayServer)
- Refactor daemon to use ChannelRegistry with per-channel-per-user agents
- Add config.get/config.patch gateway handlers (Phase 2 loose end)
- Add system.restart gateway handler (Phase 2 loose end)
- Add implementation plans and design docs

Tests: 225 passing (33 new channel adapter + gateway handler tests)

src/gateway/handlers/index.ts

@@ -6,3 +6,5 @@ export { createToolHandlers } from './tools.js';
 export type { ToolHandlerDeps } from './tools.js';
 export { createAgentHandlers } from './agent.js';
 export type { AgentHandlerDeps } from './agent.js';
+export { createConfigHandlers } from './config.js';
+export type { ConfigHandlerDeps } from './config.js';