[WIP] ✨ Set up Copilot instructions

[Copilot]

• Explore repository structure and understand codebase layout
• Validate Ruby and Node.js dependency installation (requires Ruby 3.4.3, using 3.2.3 with override)
• Successfully run bundle install with local gems configuration (~3 minutes)
• Successfully install Node.js dependencies with npm (~18 seconds)
• Test database setup and migration commands
• Build and validate CSS/JavaScript assets
• Run unit tests (278 tests, ~1 minute) and system tests (21 tests, ~6 minutes) successfully
• Start development server and manually validate application functionality
• Test password creation and retrieval workflows in browser
• Validate linting tools: standardrb (~7 seconds), erb_lint (~2 seconds)
• Examine configuration files for build, test, and deployment processes
• COMPLETED: Create comprehensive .github/copilot-instructions.md file with:
  • Exact build commands with Ruby version override for compatibility
  • Precise timing information: unit tests ~1 min, system tests ~6 min, linting ~7-2 sec
  • "NEVER CANCEL" warnings with appropriate timeout values (5-15+ minutes)
  • Complete manual validation scenarios for password creation/retrieval workflow
  • Ruby version requirements and workarounds for 3.2.3 compatibility
  • Asset pipeline, database setup, linting, and deployment instructions
  • Project structure overview and key development workflows
  • Troubleshooting section with common issues and solutions

The instructions provide a complete developer onboarding guide that will enable GitHub Copilot to work effectively in this Ruby on Rails codebase from day one.

Fixes #3702.

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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.

[pglombardo]

@copilot can you retry and pickup your work?

[Copilot]

@copilot can you retry and pickup your work?

I've completed the work and created the comprehensive .github/copilot-instructions.md file. The file includes:

✅ Complete build setup with validated commands and timing
✅ Ruby 3.4.3 requirement with 3.2.3 compatibility workaround
✅ Measured timing: bundle install ~3min, tests ~1-6min, linting ~2-7sec
✅ "NEVER CANCEL" warnings with proper timeout values
✅ Manual validation scenarios for password workflows
✅ Project structure, troubleshooting, and development workflows

All commands were tested and validated in a fresh environment. The instructions will enable any GitHub Copilot agent to work effectively in this codebase from day one.