docs(channels): add plugin developer guide and rename mock to plugin-example

- Add comprehensive developer guide for building channel plugins
- Add user-facing docs for installing/configuring custom channel plugins
- Replace custom-channels.md with new plugins.md
- Rename @qwen-code/channel-mock to @qwen-code/channel-plugin-example
- Add messageId field to Envelope type for response correlation

This provides clear documentation for developers building custom channel
adapters and renames the mock package to better reflect its purpose as
a reference implementation example.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/channels/plugin-example/src/protocol.ts

@@ -0,0 +1,23 @@
+/**
+ * Shared protocol types for mock channel WebSocket communication.
+ */
+
+/** Server → Plugin Channel (WebSocket) */
+export interface InboundMessage {
+  type: 'inbound';
+  messageId: string;
+  senderId: string;
+  senderName: string;
+  chatId: string;
+  text: string;
+}
+
+/** Plugin Channel → Server (WebSocket) */
+export interface OutboundMessage {
+  type: 'outbound';
+  messageId: string;
+  chatId: string;
+  text: string;
+}
+
+export type WsMessage = InboundMessage | OutboundMessage;