feat(channels): make plugin-example package publishable

- Update channel-base to use built dist/ output with proper exports
- Add README with quick start guide and usage instructions
- Add qwen-extension.json manifest for extension discovery
- Add start-server CLI for running the mock WebSocket server
- Update dependencies from local file: to npm version

This enables the plugin-example package to be published and installed
as a standalone extension for testing the channel plugin system.

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

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

@@ -12,5 +12,10 @@ export const plugin: ChannelPlugin = {
   displayName: 'Plugin Example',
   requiredConfigFields: ['serverWsUrl'],
   createChannel: (name, config, bridge, options) =>
-    new MockPluginChannel(name, config as MockPluginConfig, bridge, options),
+    new MockPluginChannel(
+      name,
+      config as typeof config & { serverWsUrl: string },
+      bridge,
+      options,
+    ),
 };