fix(docs): update channel plugin build backend to hatchling

The previous setuptools.backends._legacy:_Backend has been removed in
Python 3.14 and newer setuptools, causing 'Cannot import setuptools.backends.legacy' error.

Using hatchling (same as main project) ensures compatibility across Python versions.

Closes #3188

docs/CHANNEL_PLUGIN_GUIDE.md

@@ -141,8 +141,11 @@ dependencies = ["nanobot", "aiohttp"]
 webhook = "nanobot_channel_webhook:WebhookChannel"
 
 [build-system]
-requires = ["setuptools"]
-build-backend = "setuptools.backends._legacy:_Backend"
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["nanobot_channel_webhook"]
 ```
 
 The key (`webhook`) becomes the config section name. The value points to your `BaseChannel` subclass.