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/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "@qwen-code/channel-plugin-example",
+  "version": "0.1.0",
+  "type": "module",
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "dependencies": {
+    "@qwen-code/channel-base": "file:../base",
+    "ws": "^8.18.0"
+  },
+  "devDependencies": {
+    "@types/ws": "^8.5.0"
+  }
+}