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/src/SessionRouter.ts

@@ -0,0 +1,37 @@
+import type { SessionTarget } from './types.js';
+import type { AcpBridge } from './AcpBridge.js';
+
+export class SessionRouter {
+  private toSession: Map<string, string> = new Map(); // routing key → session ID
+  private toTarget: Map<string, SessionTarget> = new Map(); // session ID → target
+
+  private bridge: AcpBridge;
+  private cwd: string;
+
+  constructor(bridge: AcpBridge, cwd: string) {
+    this.bridge = bridge;
+    this.cwd = cwd;
+  }
+
+  async resolve(
+    channelName: string,
+    senderId: string,
+    chatId: string,
+    threadId?: string,
+  ): Promise<string> {
+    const key = `${channelName}:${senderId}`;
+    const existing = this.toSession.get(key);
+    if (existing) {
+      return existing;
+    }
+
+    const sessionId = await this.bridge.newSession(this.cwd);
+    this.toSession.set(key, sessionId);
+    this.toTarget.set(sessionId, { channelName, senderId, chatId, threadId });
+    return sessionId;
+  }
+
+  getTarget(sessionId: string): SessionTarget | undefined {
+    return this.toTarget.get(sessionId);
+  }
+}