docs: update Learning Hub for Copilot CLI v1.0.37–v1.0.40 changes (#1610)

- automating-with-hooks.md: Add prompt mode security gate note (v1.0.40)
  — repo hooks and workspace MCP are disabled by default in -p prompt mode;
    users must opt in via GITHUB_COPILOT_PROMPT_MODE_REPO_HOOKS and
    GITHUB_COPILOT_PROMPT_MODE_WORKSPACE_MCP env vars

- agents-and-subagents.md: Add /research orchestrator/subagent note (v1.0.40)
  and prompt mode security cross-reference for /fleet -p examples

- building-custom-agents.md: Document ACP agent list/switch capability (v1.0.40)
  — ACP clients can now list and switch custom agents via agent config option

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: 2026-02-26
+lastUpdated: 2026-05-04
 estimatedReadingTime: '10 minutes'
 tags:
   - agents
@@ -264,6 +264,8 @@ Keep agents focused—one persona per file. If you find an agent trying to do to
 
 A: In VS Code, open Copilot Chat and use the agent picker dropdown at the top of the chat panel. Your custom agents appear alongside built-in options. You can also `@mention` an agent by name.
 
+In Copilot CLI, custom agents are discoverable via the agent picker inside a session. Clients that integrate with Copilot CLI using the **Agent Coordination Protocol (ACP)** can also list available custom agents and switch between them programmatically via the `agent` session configuration option (v1.0.40+). This allows tools like Zed, Neovim plugins, and CI pipelines driving Copilot via ACP to surface the agent picker and switch agents without requiring a slash command.
+
 **Q: Can agents use skills?**
 
 A: Yes. Agents can discover and invoke skills during a conversation based on the user's intent. Skills extend what an agent can do without bloating the agent's own instructions.