docs: add Critic agent and built-in skills to course content

- Chapter 04: Add Critic agent to built-in agents table (v1.0.18)
- Chapter 05: Note that CLI now ships with built-in skills (v1.0.17)

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

Author: DanWahlin

04-agents-custom-instructions/README.md

@@ -66,6 +66,7 @@ Never used or made an agent? Here's all you need to know to get started for this
 |-------|---------------|--------------|
 | **Plan** | `/plan` or `Shift+Tab` (cycle modes) | Creates step-by-step implementation plans before coding |
 | **Code-review** | `/review` | Reviews staged/unstaged changes with focused, actionable feedback |
+| **Critic** | *Automatic* | Reviews plans and complex implementations using a second model to catch errors early (experimental, Claude models only) |
 | **Init** | `/init` | Generates project configuration files (instructions, agents) |
 | **Explore** | *Automatic* | Used internally when you ask Copilot to explore or analyze the codebase |
 | **Task** | *Automatic* | Executes commands like tests, builds, lints, and dependency installs |

05-skills/README.md

@@ -54,7 +54,9 @@ Learn what skills are, why they matter, and how they differ from agents and MCP.
    copilot
    > /skills list
    ```
-   This shows all skills Copilot can find in your project and personal folders.
+   This shows all skills Copilot can find — including any **built-in skills** that ship with the CLI itself, plus skills from your project and personal folders.
+
+   > 💡 **Built-in skills**: The Copilot CLI now comes with skills pre-installed out of the box. For example, the `customizing-copilot-cloud-agents-environment` skill provides a guide for customizing the Copilot cloud agent's environment. You don't need to create or install anything to use these — just run `/skills list` to see what's available.
 
 2. **Look at a real skill file:** Check out our provided [code-checklist SKILL.md](../.github/skills/code-checklist/SKILL.md) to see the pattern. It's just YAML frontmatter plus markdown instructions.