This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed guidance for working effectively in this Docker-based GitHub Action repository.

What's Included

The instructions provide complete guidance covering:

Bootstrap and Validation Commands

• All commands have been tested and validated to work in the current environment
• Includes fallback manual validation for network-restricted environments
• Python syntax validation, shell script checking, and YAML validation

Build Process Documentation

• Docker build process with exact timing expectations (10-20 minutes, 30+ minute timeout)
• Smoke test execution (2 minutes, 5+ minute timeout)
• Clear "NEVER CANCEL" warnings for long-running operations

Manual Validation Scenarios

• Step-by-step testing procedures for Python scripts, shell scripts, and action definitions
• Functional testing scenarios for hash calculation and package name extraction
• Security considerations for credential handling

Comprehensive Troubleshooting

• Documents known network limitations in sandboxed environments
• Provides workarounds when automated tools fail due to SSL/connectivity issues
• Explains why Docker builds and pip installations may fail in restricted environments

Repository Structure Guide

• Detailed explanation of key files and their purposes
• Dependencies and requirements management with pip-tools
• Development best practices and validation workflows

Key Features

• Firm directive to always reference instructions first before searching or using bash commands
• Exhaustively validated - every documented command has been tested to work correctly
• Handles limitations - provides manual alternatives when network restrictions prevent automated tools
• Security-aware - includes guidance for handling sensitive credentials and OIDC flows
• Developer-friendly - structured for quick reference and efficient workflow

The instructions enable any developer to quickly understand this codebase and work effectively while navigating the common challenges of sandboxed development environments.

Fixes #374.

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.