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

- Chapter 04: Add Critic agent to the built-in agents table (v1.0.18)
- Chapter 05: Add built-in CLI skills as a third skill location (v1.0.17)

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

Author: DanWahlin

04-agents-custom-instructions/README.md

@@ -69,6 +69,7 @@ Never used or made an agent? Here's all you need to know to get started for this
 | **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 |
+| **Critic** | *Automatic (experimental)* | Automatically reviews plans and complex implementations using a complementary model to catch errors early. Available when using Claude models. |
 
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05-skills/README.md

@@ -277,6 +277,7 @@ Copilot automatically scans these locations for skills:
 |----------|-------|
 | `.github/skills/` | Project-specific (shared with team via git) |
 | `~/.copilot/skills/` | User-specific (your personal skills) |
+| Built into the CLI | Bundled with the CLI itself (e.g., a guide for customizing the Copilot cloud agent environment) |
 
 ### Skill Structure