Add comprehensive GitHub Copilot instructions for vals 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 vals codebase.

Key Features

The instructions include:

Validated Build & Test Commands:

• Exact build commands with verified timing (5-6s normal, 3-4min with dependencies)
• Core test commands that work without external credentials
• Linting setup with proper golangci-lint installation
• Critical "NEVER CANCEL" warnings with specific timeout values (300s+ for builds, 180s+ for linting)

Comprehensive Validation Scenarios:

• 10 basic validation tests covering all major CLI functionality
• Error handling validation for invalid providers and missing files
• Tests using providers that work without external dependencies (echo, file, envsubst)

Developer Workflow Guidance:

• Repository structure navigation with key directories explained
• 28+ provider ecosystem overview
• Development patterns and debugging tips
• Pre-commit validation requirements

Manual Testing Examples:

# Basic functionality test
echo 'test: ref+echo://hello-world' | ./bin/vals eval -f -

# File provider test with existing test files
echo 'test: ref+file://./myjson.json#/baz/mykey' | ./bin/vals eval -f -

# Environment variable substitution
export VAR1=hello-world && echo 'test: ref+envsubst://$VAR1' | ./bin/vals eval -f -

All commands and scenarios have been thoroughly tested and validated to ensure they work correctly for any developer or AI agent starting fresh with this repository.

The instructions start with a firm directive to follow the provided guidance before falling back to additional search, as requested in the requirements.

Fixes #743.

Warning

Firewall rules blocked me from connecting to one or more addresses

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

• metadata.google.internal
• vals-test.vault.azure.net
  • Triggering command: /tmp/go-build2678667348/b1767/vals.test -test.paniconexit0 -test.gocoverdir=/tmp/go-build2678667348/b1767/gocoverdir -test.timeout=10m0s -test.v=true -test.coverprofile=/tmp/go-build2678667348/b1767/_cover_.out (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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