Add GitHub Copilot instructions for repository standards and workflow

[Copilot]

Establishes .github/copilot-instructions.md to guide Copilot contributions with repository-specific standards and patterns.

Changes

• Code standards: make fmt requirement before commits, build/lint/swagger workflow
• Repository structure: Documents actual directory layout (cmd/, internal/, pkg/) with verified paths
• Development guidelines: Go idioms, table-driven tests, Swagger annotations, cloud provider patterns
• Build system: Clarifies Makefile vs Makefile.new usage, provides manual test commands (go test -v -race ./...)
• Security & conventions: OWASP guidelines, structured errors, dependency injection patterns

Key Sections

### Required Before Each Commit
- Run `make fmt` before committing any changes

### Go Best Practices
1. Use table-driven tests when possible
2. Use the race detector (`-race` flag)
3. Document public APIs using godoc conventions

### API Development
1. Use Swagger annotations for all HTTP endpoints
2. Run `make swagger` after updating handlers

Instructions reflect the actual codebase structure as an infrastructure auditing platform for AWS, Azure, and GCP.

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>This is a Go based repository with a Ruby client for certain API endpoints. It is primarily responsible for ingesting metered usage for GitHub and recording that usage. Please follow these guidelines when contributing:

Code Standards

Required Before Each Commit

• Run make fmt before committing any changes to ensure proper code formatting
• This will run gofmt on all Go files to maintain consistent style

Development Flow

• Build: make build
• Test: make test
• Full CI check: make ci (includes build, fmt, lint, test)

Repository Structure

• cmd/: Main service entry points and executables
• internal/: Logic related to interactions with other GitHub services
• lib/: Core Go packages for billing logic
• admin/: Admin interface components
• config/: Configuration files and templates
• docs/: Documentation
• proto/: Protocol buffer definitions. Run make proto after making updates here.
• ruby/: Ruby implementation components. Updates to this folder should include incrementing this version file using semantic versioning: ruby/lib/billing-platform/version.rb
• testing/: Test helpers and fixtures

Key Guidelines

1. Follow Go best practices and idiomatic patterns
2. Maintain existing code structure and organization
3. Use dependency injection patterns where appropriate
4. Write unit tests for new functionality. Use table-driven unit tests when possible.
5. Document public APIs and complex logic. Suggest changes to the docs/ folder when appropriate</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #12

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