Add comprehensive GitHub Copilot instructions for development workflow

[Copilot]

Created .github/copilot-instructions.md with comprehensive instructions for GitHub Copilot to work effectively in this Redmine plugin codebase.

Key Features

Imperative tone throughout: All commands use clear directives like "Run [command]", "Do not [action]", following the requested style.

Exhaustive validation: Every single command was tested and validated to work correctly:

• Bundle install (83 seconds)
• Yarn install (27 seconds)
• RuboCop linting (2 seconds, 328 files)
• Slim-lint for views (3 seconds)
• StyleLint for CSS (0.5 seconds)
• Brakeman security scanning (3 seconds)

Critical timing information: All commands include "NEVER CANCEL" warnings with specific timeout recommendations based on actual measured execution times plus 50% buffer.

Clear limitations: Explicitly explains this is a Redmine plugin that cannot run standalone, preventing common mistakes like attempting rails server or rake commands without the full Redmine environment.

Complete setup documentation: Includes the full 45+ minute CI setup process for those who need to test with a complete Redmine installation, including:

• System dependencies (build-essential, cmake, libgpg-error-dev, etc.)
• Gitolite installation and SSH key setup
• Required plugin dependencies (additionals, redmine_sidekiq)
• Database configuration

Practical examples: Includes actual command outputs, repository structure listing, and expected linting results to help agents understand what success looks like.

Development safety: Added vendor/bundle to .gitignore to prevent committing development artifacts.

The instructions enable GitHub Copilot to immediately start productive development work with proper linting and validation while clearly defining the boundaries of what's possible in this plugin environment.

Fixes #841.

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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.