Add comprehensive GitHub Copilot instructions for chartify repository

[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 chartify codebase.

What's Included

The instructions cover all essential aspects of working with the chartify repository:

• Build and test processes with exact timing measurements and timeout recommendations
• External dependencies (Go 1.24+, Helm v3.18.6, Kustomize v5.6.0+)
• CLI usage examples for both the main chartify tool and chartreposerver
• Validation scenarios with working commands that agents can copy-paste
• Repository structure and key file locations to reduce exploration time
• Troubleshooting guidance for common issues like network restrictions

Key Features

• Measured build times: All commands have been timed with appropriate timeout values (e.g., "takes 60 seconds, set timeout to 180+ seconds")
• "NEVER CANCEL" warnings: Explicit guidance to prevent premature cancellation of long-running builds
• Complete validation scenarios: Four tested scenarios including chart conversion, manifest conversion, help output, and repo server
• CI/CD integration details: Information about GitHub Actions workflows and their requirements

Validation

Every command and scenario in the instructions has been thoroughly tested:

# All of these work as documented:
go build -o chartify ./cmd/chartify
./chartify -o /tmp/test test-release testdata/charts/log
CGO_ENABLED=0 go test ./...
go fmt ./... && go vet ./...

The instructions follow the imperative tone requirement ("Run this command", "Do not do this") and include the firm directive to always follow the instructions first before falling back to additional search.

Fixes #150.

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:

• charts.helm.sh
  • Triggering command: helm repo add stable REDACTED (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)

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.