Add copilot-setup-steps.yml for Copilot coding agent environment

[Copilot]

Adds the required .github/workflows/copilot-setup-steps.yml so GitHub Copilot coding agent has a fully functional Zephyr RTOS development environment pre-provisioned before it starts working.

Dependencies installed

• System packages: cmake, ninja-build, device-tree-compiler, gperf, ccache, dfu-util, perl, doxygen, graphviz, and supporting libs — mirrors what the existing checkpatch.yml and sedi_docs_update.yml workflows rely on
• Python + west: Zephyr's meta-build tool installed via pip with $HOME/.local/bin added to PATH
• Zephyr workspace: Full workspace initialized via west init (zephyrproject-rtos/zephyr main) + west update + Python requirements
• Zephyr SDK: Downloaded to $HOME/zephyr-sdk at the version pinned by zephyr/SDK_VERSION; all toolchains installed (-t all)

Notes

• Runs on ubuntu-22.04 consistent with existing CI workflows
• SDK installed to user-writable $HOME/zephyr-sdk (no sudo required)
• SDK_VERSION file existence is validated before download with an explicit error on failure
• Workflow triggers on workflow_dispatch, push, and pull_request scoped to this file for easy iterative validation

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.