Plugin authoring guide exists

Without a plugin authoring guide, a developer who wants to extend the system must reverse-engineer existing plugins and read the host's internal source code. This creates a barrier that collapses the potential plugin ecosystem to however many developers are willing to invest that archaeology time — typically zero third-party developers. ISO 25010 maintainability.analysability scores fall when the system cannot be understood without execution; an authoring guide is the primary mechanism for making the plugin API understandable before writing code.

Create a docs/plugins.md or docs/plugin-authoring-guide.md that covers the complete authoring workflow from manifest creation through testing. Include a working code example for each major step.

# Creating a Plugin

## 1. Create the manifest
Create a `plugin.json` file with `name`, `version`, and `engines.host-api`.

## 2. Implement the plugin
Export a class implementing the `Plugin` interface from `'host/plugin-api'`.

## 3. Register hooks
In your `activate()` method, call `context.hooks.on('before:request', handler)`.

## 4. Test your plugin
Use `createTestHost()` from `'host/testing'` to run your plugin in isolation.

## 5. Distribute
Publish to npm with the `my-app-plugin` keyword.