feat: add GitHub Copilot platform support

[GuGuGuGun]

Summary

• Add GitHub Copilot as a supported platform in CLI initialization flow
• Introduce Copilot configurator and platform template assets
• Wire platform selection and extraction logic to include Copilot
• Keep existing platform behaviors unchanged

Motivation

Trellis currently supports multiple AI coding platforms. This change adds first-class GitHub Copilot support so users can initialize and use Trellis workflows consistently in Copilot environments.

Changes

• CLI
  • Extend platform type definitions and init command handling for Copilot
  • Update configurator registry and platform routing
• Templates
  • Add Copilot template files and hook/session-start assets
  • Ensure extract flow includes Copilot template packaging
• Adapter scripts
  • Update CLI adapter paths/logic to recognize Copilot integration points

Validation

• Passed pnpm lint
• Passed pnpm typecheck
• Built and pushed successfully on branch feat/copilot-platform
• Verified no unintended local-only config file was committed

Compatibility / Risk

• Low risk for existing platforms: changes are additive and platform-scoped
• Main regression surface: platform selection/init path and template extraction
• Recommended reviewer focus: init command routing and template copy/extract behavior for all existing platforms

Checklist

• Conventional Commits followed
• Lint and typecheck passed
• Branch pushed to fork
• Ready for review against upstream main