docs: add April 2026 Copilot CLI feature updates

- Add /env slash command to ch01 reference table (v1.0.25)
- Add --plan and --mode startup flags note to ch01 (v1.0.23)
- Add skills field to agent YAML properties table in ch04 (v1.0.22)
- Add MCP registry install (copilot mcp install) to ch06 (v1.0.25)

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

Author: DanWahlin

01-setup-and-first-steps/README.md

@@ -320,6 +320,8 @@ Proceed with implementation? [Y/n]
 
 > 📚 **Autopilot mode**: You may have noticed Shift+Tab cycles through a third mode called **Autopilot**. In autopilot mode, Copilot works through an entire plan without waiting for your input after each step — like handing a task to a colleague and saying "let me know when you're finished." The typical workflow is plan → accept → autopilot, which means you need to be good at writing plans first. You can also launch directly into autopilot with `copilot --autopilot`. Get comfortable with Interactive and Plan modes first, then see the [official docs](https://docs.github.com/copilot/concepts/agents/copilot-cli/autopilot) when you're ready.
 
+> 💡 **Start in a specific mode**: You can launch Copilot directly in any mode using flags: `copilot --plan` starts in Plan mode, `copilot --autopilot` starts in Autopilot mode, and `copilot --mode interactive` (or `plan` / `autopilot`) lets you specify the mode explicitly. This is handy when you already know what you want to do.
+
 ---
 
 ### Mode 3: Programmatic Mode
@@ -445,6 +447,7 @@ That's it for getting started! As you become comfortable, you can explore additi
 | Command | What It Does |
 |---------|--------------|
 | `/changelog` | Display changelog for CLI versions |
+| `/env` | Show your loaded environment (instructions, MCP servers, skills, agents, plugins) |
 | `/feedback` | Submit feedback to GitHub |
 | `/help` | Show all available commands |
 | `/theme` | View or set terminal theme |

04-agents-custom-instructions/README.md

@@ -131,7 +131,7 @@ When reviewing code, always check for:
 - Hardcoded secrets
 ```
 
-> 💡 **Required vs Optional**: The `description` field is required. Other fields like `name`, `tools`, and `model` are optional.
+> 💡 **Required vs Optional**: The `description` field is required. Other fields like `name`, `tools`, `model`, and `skills` are optional.
 
 ## Where to put agent files
 
@@ -507,8 +507,12 @@ You are a Python specialist focused on code quality and best practices.
 | `name` | No | Display name (defaults to filename) |
 | `description` | **Yes** | What the agent does - helps Copilot understand when to suggest it |
 | `tools` | No | List of allowed tools (omit = all tools available). See tool aliases below. |
+| `model` | No | AI model to use for this agent (e.g., `claude-sonnet-4.5`) |
+| `skills` | No | List of skill names to eagerly load into the agent's context at startup |
 | `target` | No | Limit to `vscode` or `github-copilot` only |
 
+> 💡 **The `skills` field**: When you add a `skills` list to an agent, those skills are automatically loaded when the agent starts — you don't need to invoke them manually. This is useful when your agent always needs specific instructions (e.g., a test-writer agent that always loads your `pytest-gen` skill). See [Chapter 05](../05-skills/README.md) for how to create skills.
+
 ### Tool Aliases
 
 Use these names in the `tools` list:

06-mcp-servers/README.md

@@ -919,6 +919,8 @@ copilot mcp enable filesystem
 copilot mcp disable context7
 ```
 
+> 💡 **Install from the registry**: You can also install MCP servers from the official registry with guided setup. Run `copilot mcp install` and Copilot will walk you through picking a server and configuring it interactively — no manual JSON editing required. This is the easiest way to add a new server you've heard about.
+
 > 💡 **When to use which?** Use `/mcp` slash commands when you're already in a chat session. Use `copilot mcp` from the terminal when you want to quickly check or change your server settings before starting a session.
 
 For most of this course, `/mcp show` is all you need. The other commands become useful as you manage more servers over time.