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/tsconfig.json

@@ -0,0 +1,10 @@
+{
+  "extends": "../../../tsconfig.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": "src"
+  },
+  "include": ["src/**/*.ts"],
+  "exclude": ["node_modules", "dist"],
+  "references": [{ "path": "../base" }]
+}