Add .github/copilot-instructions.md with Meshtastic design standards

[Copilot]

Adds Copilot instructions file so GitHub Copilot automatically references the Meshtastic Client Design Standards when assisting with UI changes, per the recommendation in the meshtastic/design repo README.

• Creates .github/copilot-instructions.md with a directive to fetch and review the canonical design standards document before making UI changes

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