[bot] Add Critic agent and built-in skills documentation (v1.0.17–v1.0.18)

[DanWahlin]

What's New

This PR updates course chapters 04 and 05 to reflect new Copilot CLI features released in the past 7 days (v1.0.15–v1.0.18, April 1–4 2026).

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New Features Found

v1.0.18 (April 4, 2026)

• Critic agent — A new built-in agent that automatically reviews plans and complex implementations using a complementary model to catch errors early. Available in experimental mode for Claude models.

v1.0.17 (April 3, 2026)

• Built-in skills shipped with the CLI — The CLI now includes built-in skills out of the box. The first is a guide for customizing the Copilot cloud agent's environment (customizing-copilot-cloud-agents-environment), helpful for teams using copilot-setup-steps.yml.

Note: /share html, /mcp auth, and camelCase config settings from v1.0.15 were already documented in the course.

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Sections Updated

Chapter 04 — Agents and Custom Instructions

• Added Critic agent row to the Built-in Agents table with a description and note that it requires --experimental and Claude models.

Chapter 05 — Skills System

• Updated the "New to Skills? Start Here!" section to mention built-in skills and give learners context about /skills list showing them automatically.
• Updated the Skill Locations table to include a "Built-in" row explaining these are always available with no setup.

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Source Announcements

• v1.0.18 Release
• v1.0.17 Release

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