Set up Copilot coding agent instructions and environment pre-configuration

[Copilot]

Adds the two files recommended by GitHub's Copilot coding agent best practices to help the agent produce better PRs for this repository.

Changes

• .github/copilot-instructions.md — Repository-wide context for the agent: project overview, source layout, exact xcodebuild commands for debug builds and tests, Objective-C/ARC coding conventions (TT/CSV class prefixes, NSError ** error handling, XCTest patterns), and key constraints (deployment targets, no external package managers, AppKit-only UI).

• .github/copilot-setup-steps.yml — Pre-warms the agent's ephemeral macOS environment before it starts a task: confirms Xcode availability and runs a full debug build to populate the derived data cache, so the agent can build and run tests immediately without trial-and-error environment discovery.

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #5

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