Add Xcode installation guide for Codex and Claude Agent

[RossTarrant]

Summary

Adds a new install-xcode.md installation guide covering how to add the GitHub MCP server to Xcode's built-in coding agents (Codex and Claude Agent).

Why

There was no documentation for adding the GitHub MCP server to Xcode's native coding agents, making setup unclear for users. This addresses the request raised in #2120.

Fixes #2120

What changed

• Added docs/installation-guides/install-xcode.md — a focused guide explaining Xcode's agent-specific config directories, linking to existing install-codex.md and install-claude.md for setup steps, and noting the macOS PATH requirement for local STDIO servers
• Updated docs/installation-guides/README.md — added install-xcode.md to the guides list and updated the support table to show both Xcode (Codex) and Xcode (Claude Agent) with local and remote support

MCP impact

• No tool or API changes

Prompts tested (tool changes only)

• N/A — documentation only

Security / limits

• No security or limits impact

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

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

Docs

• Updated (README / docs / examples)

[Copilot]

Pull request overview

Adds documentation for configuring the GitHub MCP Server inside Xcode’s built-in coding agents, so users can follow a clear, Xcode-specific setup path rather than relying on global CLI/desktop configuration locations.

Changes:

• Added a new Xcode installation guide describing agent-specific configuration directories and PATH caveats.
• Updated the installation-guides index to include the Xcode guide and list Xcode (Codex) / Xcode (Claude Agent) in the support table.

Show a summary per file

File	Description
docs/installation-guides/README.md	Adds the Xcode guide to the list and adds Xcode (Codex/Claude Agent) rows to the support matrix.
docs/installation-guides/install-xcode.md	New guide documenting Xcode-specific config directories, setup links, PATH notes, and troubleshooting.

Copilot's findings

Comments suppressed due to low confidence (1)

docs/installation-guides/install-xcode.md:34

• The troubleshooting guidance references ~/.claude.json, but the existing Claude guide documents different default locations (e.g., ~/Library/Application Support/Claude/claude_desktop_config.json on macOS). To prevent confusion, update this to reference the same defaults described in install-claude.md (or the correct global path for the specific Claude Agent you mean).

| Tools not loading | Config placed in wrong directory | Ensure config is in the Xcode-specific path above, not `~/.codex/` or `~/.claude.json` |

• Files reviewed: 2/2 changed files
• Comments generated: 3

[SamMorrowDrums]

Thanks @RossTarrant