Add GitHub Action for go-covercheck

[Copilot]

This PR adds an officially supported GitHub Action that makes it easy to integrate go-covercheck into CI/CD workflows.

What's Added

GitHub Action (action.yml)

• Composite action using go install for fast, reliable installation
• Comprehensive input support for all major go-covercheck flags:
  • Coverage file path, config file, thresholds
  • Output formats, skip patterns, history functionality
  • Version pinning support
• Proper argument handling with validation and error checking
• PATH management to ensure installed binary is accessible

Documentation & Examples

• Updated README with complete GitHub Action documentation
• Input parameter reference with descriptions and defaults
• Multiple usage examples covering common scenarios:
  • Basic usage with default settings
  • Custom thresholds and output formats
  • Skip patterns and history tracking
• Sample workflow file (samples/github-action-workflow.yml) for easy copying

Testing Infrastructure

• Test workflow (.github/workflows/test-action.yml) to validate action functionality
• Multiple test scenarios covering different input combinations
• Local validation to ensure script works correctly

Usage

steps:
  - uses: actions/checkout@v4
  - uses: actions/setup-go@v5
    with:
      go-version: '1.24'
  - run: go test -v -coverprofile=coverage.out ./...
  - uses: mach6/go-covercheck@v1
    with:
      coverage-file: coverage.out
      statement-threshold: "80"
      block-threshold: "70"

The action leverages the existing go install approach used in the project's CI, ensuring consistency and reliability while providing a clean, user-friendly interface for GitHub Actions workflows.

Fixes #15.

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