docs: update Learning Hub for CLI v1.0.6–v1.0.10 changes (#1119)

- automating-with-hooks: add subagentStart event to hook events table,
  add cross-platform PascalCase event name compatibility note, add
  subagentStart example, expand hook config file location FAQ to include
  settings.json and config.json locations
- defining-custom-instructions: document that applyTo accepts both
  string and array formats (added in v1.0.6)
- installing-and-using-plugins: document --plugin-dir flag and the
  External Plugins section in /plugin list (added in v1.0.10)

Source: github/copilot-cli releases v1.0.6 through v1.0.10
(March 16–20, 2026)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/content/docs/learning-hub/installing-and-using-plugins.md

@@ -3,7 +3,7 @@ title: 'Installing and Using Plugins'
 description: 'Learn how to find, install, and manage plugins that extend GitHub Copilot CLI with reusable agents, skills, hooks, and integrations.'
 authors:
   - GitHub Copilot Learning Hub Team
-lastUpdated: 2026-02-26
+lastUpdated: 2026-03-22
 estimatedReadingTime: '8 minutes'
 tags:
   - plugins
@@ -177,6 +177,16 @@ copilot plugin update my-plugin
 copilot plugin uninstall my-plugin
 ```
 
+### Loading Plugins from a Local Directory
+
+You can load plugins directly from a local directory without installing them from a marketplace, using the `--plugin-dir` flag when starting Copilot:
+
+```bash
+copilot --plugin-dir /path/to/my-plugin
+```
+
+Plugins loaded this way appear in `/plugin list` under a separate **External Plugins** section, clearly distinguished from marketplace-installed plugins. This is useful for testing local plugins in development or loading private plugins that aren't published to any marketplace.
+
 ### Where Plugins Are Stored
 
 - **Marketplace plugins**: `~/.copilot/installed-plugins/MARKETPLACE/PLUGIN-NAME/`