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>

package-lock.json

@@ -18994,7 +18994,7 @@
       "name": "@qwen-code/channel-plugin-example",
       "version": "0.1.0",
       "dependencies": {
-        "@qwen-code/channel-base": "file:../base",
+        "@qwen-code/channel-base": "^0.1.0",
         "ws": "^8.18.0"
       },
       "devDependencies": {