docs(learning-hub): update for Copilot CLI v1.0.64-v1.0.65 features (#2127)

- creating-effective-skills: add 'copilot skill' CLI subcommand for
  listing, adding, and removing skills (v1.0.65); add '/skill' alias
- automating-with-hooks: document userPromptSubmitted additionalContext
  injection into model-facing prompt (v1.0.65); update hook events table
- copilot-configuration-basics: add opt-in CI check status bar indicator
  for current branch (v1.0.65)
- building-custom-agents: add note that /security-review is now available
  to all users without --experimental (v1.0.64)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/content/docs/learning-hub/building-custom-agents.md

@@ -3,7 +3,7 @@ title: 'Building Custom Agents'
 description: 'Learn how to create specialized GitHub Copilot agents with custom personas, tool integrations, and domain expertise.'
 authors:
   - GitHub Copilot Learning Hub Team
-lastUpdated: 2026-05-05
+lastUpdated: 2026-06-25
 estimatedReadingTime: '10 minutes'
 tags:
   - agents
@@ -186,6 +186,8 @@ You are a release manager who automates the release process.
 
 Create agents that enforce standards:
 
+> **Built-in `/security-review`**: Before creating a custom security-reviewer agent, note that GitHub Copilot CLI includes a built-in `/security-review` command (available to all users since v1.0.64). It performs a security-focused analysis of staged changes or specified files. Custom security-reviewer agents are still valuable for domain-specific rules, team conventions, and deep integration with MCP tools like Sentry or SAST platforms.
+
 ```markdown
 ---
 name: 'Accessibility Auditor'