feat(channels): add Telegram channel integration with ACP bridge

Implements the channels infrastructure for connecting external messaging
platforms to Qwen Code via ACP. Phase 1 supports plain text round-trip:
Telegram user sends message -> AcpBridge -> qwen-code --acp -> response
back to Telegram.

New packages:
- @qwen-code/channel-base: AcpBridge, SessionRouter, SenderGate, ChannelBase
- @qwen-code/channel-telegram: TelegramAdapter using telegraf

CLI: `qwen channel start <name>` reads from settings.json channels config,
spawns ACP agent, connects to Telegram via polling.

packages/channels/base/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "@qwen-code/channel-base",
+  "version": "0.1.0",
+  "description": "Base channel infrastructure for Qwen Code",
+  "type": "module",
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "dependencies": {
+    "@agentclientprotocol/sdk": "^0.14.1"
+  },
+  "devDependencies": {
+    "typescript": "^5.0.0"
+  }
+}