feat(cli): add opt-in Copilot setup workflow for create

[jong-kyung]

Summary

Adds opt-in GitHub Copilot Coding Agent setup during vp create.
When users explicitly select Copilot via --agent copilot, generated projects now include .github/workflows/copilot-setup-steps.yml so Copilot can set up Vite+ and run vp directly.

Changes

• Add Copilot setup workflow generation for explicit Copilot agent selection
• Keep default scaffolds clean
  • no workflow for default vp create
  • no workflow for --no-agent
  • no workflow for non-Copilot agents
• Preserve existing workflow files without overwriting them
• Refactor agent selection metadata to return selected supported agent definitions
• Add snap coverage for default, non-Copilot, no-agent, and Copilot create flows

closes #1336

[netlify]

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[jong-kyung]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

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