Add Copilot agent setup: copilot-instructions, setup-steps workflow, and rtk init hook

[Copilot]

This repo was missing the two files present in every other kubb-labs project that make the Copilot agent environment functional and token-efficient.

Changes

• .github/copilot-instructions.md — adds RTK instruction file so the agent knows to prefix all shell commands with rtk (otherwise RTK is installed but never used)
• .github/workflows/copilot-setup-steps.yml — adds the required Copilot pre-provisioning workflow; job must be named copilot-setup-steps for GitHub to pick it up; delegates to the existing .github/setup/action.yml composite action so RTK install is included
• .github/setup/action.yml — adds rtk init -g --copilot step after RTK install to wire up the shell hook, enabling transparent command proxying (git status → rtk git status) during agent sessions

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Latest commit: ae4889a

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