[bot] Document Copilot app Customize skill and plugin management

[github-actions]

Summary

Adds documentation for GitHub Copilot app features released in v1.1.9 and v1.1.10 that were not yet covered in the Learning Hub:

• Personal skill creation from Customize (v1.1.9+): Users can now create, edit, and remove personal skills directly from the Copilot app's Customize screen, with Markdown preview and validation.
• Plugin update management from Customize (v1.1.9+): Installed plugins can be updated individually or all at once from Customize or Settings, with installed version visible.
• Automatic plugin updates from the app (v1.1.10+): Plugins and their extensions can now be set to auto-update directly from Customize → Plugins.

Sections updated

• website/src/content/docs/learning-hub/github-copilot-app.md — new subsection under Key Features describing Customize-based skill and plugin management.
• website/src/content/docs/learning-hub/creating-effective-skills.md — callout noting the GUI-based skill creation option in the Copilot app.
• website/src/content/docs/learning-hub/installing-and-using-plugins.md — callout describing plugin management/auto-update via the Copilot app, alongside the existing CLI-based auto-update documentation.

All three pages' lastUpdated fields were bumped to reflect the change.

Source announcements

• https://github.com/github/app/blob/main/changelog.md (v1.1.9, v1.1.10 releases of the GitHub Copilot desktop app)

Other recent CLI/app changes (e.g. new model support, sandbox auth tab restructuring, worktreeBaseRef setting) were reviewed but found to already be documented in copilot-configuration-basics.md, so no further edits were made there. cli-for-beginners pages were intentionally left untouched per the maintenance process for that section.

Warning

Firewall blocked 2 domains

The following domains were blocked by the firewall during workflow execution:

• code.visualstudio.com
• github.blog

To allow these domains, add them to the network.allowed list in your workflow frontmatter:

network:
  allowed:
    - defaults
    - "code.visualstudio.com"
    - "github.blog"

See Network Configuration for more information.

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