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/qwen-extension.json

@@ -0,0 +1,10 @@
+{
+  "name": "qwen-channel-plugin-example",
+  "version": "0.1.0",
+  "channels": {
+    "plugin-example": {
+      "entry": "dist/index.js",
+      "displayName": "Plugin Example Channel"
+    }
+  }
+}