Improve Copilot CLI installation doc

[kerobbi]

Summary

This PR restructures the Copilot CLI installation guide to reflect that the server comes pre-installed. It also adds CLI flags documentation, server naming behaviour, and fixes the provided remote config.

Why

• The original doc assumed users needed to configure the server from scratch.
• The provided remote config was missing the type field

What changed

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs only change.

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

This PR updates the Copilot CLI installation guide to reflect that the GitHub MCP server is available as a built-in server in Copilot CLI, and expands the guide with additional configuration and verification details.

Changes:

• Documented the built-in github-mcp-server in Copilot CLI and added per-session CLI flag examples.
• Clarified custom configuration options and server naming behavior.
• Fixed the remote server config example by adding the missing type field and added a reference link for remote options.

[tommaso-moro]

Looks good to me! Only comment is that we could add a link to our server configuration docs as it goes in depth on things like read only mode and lockdown mode? Just a thought! Not a blocker at all, I think this is good to go