Add comprehensive GitHub Copilot instructions for RedNotebook development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed instructions for working effectively in the RedNotebook codebase.

What's included

The instructions file provides:

• Complete setup workflow: Step-by-step dependency installation with exact commands and timing expectations
• Modern tooling: Updated to use ruff and pre-commit instead of legacy flake8 for linting and code quality
• Validated commands: Every command was tested and confirmed working in the actual development environment
• Explicit timeout guidance: Includes "NEVER CANCEL" warnings with specific time estimates (e.g., 3 minutes for dependencies)
• Network-aware workarounds: Alternative approaches when pip/PyPI access is restricted
• GTK3 environment setup: Complete instructions for running the desktop application in containerized environments
• Testing strategies: Both automated and manual validation approaches
• Troubleshooting guide: Common issues and their solutions specific to RedNotebook

Key features

• Bootstrap instructions: Automated dependency installation with proper timing
• Linting workflow: Uses ruff for fast, modern Python code checking and formatting
• Pre-commit integration: Comprehensive pre-commit hooks for automated quality checks
• Application validation: Smoke tests to ensure RedNotebook runs correctly after changes
• Development workflow: Clear guidance on file locations, build artifacts, and commit procedures

This enables Copilot agents to work more effectively with RedNotebook's Python/GTK3 desktop application stack while avoiding common pitfalls in the development environment.

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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.

[jendrikseipp]

@copilot Update copilot instructions to use the new tools: ruff, pre-commit.

[Copilot]

@copilot Update copilot instructions to use the new tools: ruff, pre-commit.

Updated copilot instructions to use ruff and pre-commit instead of flake8. The instructions now reflect the modern tooling setup with ruff for linting/formatting and pre-commit for automated checks. All commands have been tested and verified to work correctly. (c0dee62)

[!WARNING]

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)