Add comprehensive GitHub Copilot instructions

[Copilot]

• Create .github/copilot-instructions.md with comprehensive instructions covering:
  • Project overview and purpose
  • Complete repository structure map
  • Architecture (two-phase compilation + rendering pipeline, key types table)
  • Coordinate system details
  • How to implement new TurtleCommand and Instruction classes
  • Dynamic parameter pattern conventions
  • Control-flow command patterns
  • Complete list of all available commands with descriptions
  • Code style guidelines
  • Bootstrap and Build section (flutter pub get, flutter build apk --debug, flutter build web --wasm)
  • Testing and linting instructions
  • Dependencies table

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[zonble]

@copilot The build fails. Fix it.

[Copilot]

@copilot The build fails. Fix it.

The failure in the previous run was a transient GitHub Actions infrastructure error — runner 1000003738 returned a 401 Unauthorized when downloading the actions/setup-java action during "Set up job" for the 3.29.x matrix entry. All 5 other matrix jobs succeeded. There is no code or build issue.

Pushed commit 1a893f4 (which adds a "Bootstrap and Build" section to the instructions) to trigger a fresh CI run that should pass cleanly.