feat(channels): add ChannelPlugin interface and registry-based factory

Refactor channel system to use a formal plugin interface:

- Add ChannelPlugin interface to @qwen-code/channel-base (channelType,
  displayName, requiredConfigFields, createChannel factory)
- Each built-in adapter (Telegram, WeChat, DingTalk) exports a plugin object
- Replace hardcoded if/else factory with a Map-based channel registry
- Config validation now uses plugin's requiredConfigFields dynamically
- ChannelType changed from fixed union to string for extensibility
- Multi-channel mode now picks model from first channel config

This is the foundation for external plugin support — built-in channels
go through the exact same code path that third-party plugins will use.

packages/channels/base/src/index.ts

@@ -15,6 +15,7 @@ export type { SenderCheckResult } from './SenderGate.js';
 export { SessionRouter } from './SessionRouter.js';
 export type {
   ChannelConfig,
+  ChannelPlugin,
   ChannelType,
   Envelope,
   GroupConfig,

packages/channels/base/src/types.ts

@@ -1,11 +1,9 @@
+import type { AcpBridge } from './AcpBridge.js';
+import type { ChannelBase, ChannelBaseOptions } from './ChannelBase.js';
+
 export type SenderPolicy = 'allowlist' | 'pairing' | 'open';
 export type SessionScope = 'user' | 'thread' | 'single';
-export type ChannelType =
-  | 'telegram'
-  | 'weixin'
-  | 'dingtalk'
-  | 'discord'
-  | 'webhook';
+export type ChannelType = string;
 export type GroupPolicy = 'disabled' | 'allowlist' | 'open';
 
 export interface GroupConfig {
@@ -52,3 +50,30 @@ export interface SessionTarget {
   chatId: string;
   threadId?: string;
 }
+
+/**
+ * A channel plugin registers a channel type and provides a factory
+ * to create adapter instances. Both built-in adapters and external
+ * plugins conform to this interface.
+ */
+export interface ChannelPlugin {
+  /** Unique channel type ID (e.g., "telegram", "tmcp-dingtalk"). */
+  channelType: string;
+
+  /** Human-readable name for CLI output. */
+  displayName: string;
+
+  /**
+   * Config fields required by this channel type, beyond the shared
+   * ChannelConfig fields. Validated at startup.
+   */
+  requiredConfigFields?: string[];
+
+  /** Create a channel adapter instance. */
+  createChannel(
+    name: string,
+    config: ChannelConfig & Record<string, unknown>,
+    bridge: AcpBridge,
+    options?: ChannelBaseOptions,
+  ): ChannelBase;
+}