feat: set up code coverage reporting with Codecov

[Pertempto]

Description

Implement comprehensive code coverage reporting for the simple-sync project using Codecov. This will provide visibility into test coverage, help maintain code quality, and enable coverage-based pull request checks.

Acceptance Criteria

• Set up Codecov account and repository integration
• Configure CI workflow to generate and upload coverage reports
• Add coverage badges to README.md
• Set up coverage thresholds and quality gates
• Configure branch protection rules for coverage requirements
• Enable coverage reporting for pull requests
• Set up coverage trend monitoring and alerts

Technical Requirements

• Use Codecov's GitHub Actions integration
• Generate coverage reports using Go's built-in coverage tools
• Support coverage for unit tests, contract tests, and future integration tests
• Configure coverage exclusion for generated files, test files, and third-party code
• Set up coverage targets (e.g., 80% overall coverage, no new code below 70%)
• Enable coverage reporting for all active branches (main, develop, feature branches)

Implementation Steps

1. Codecov Account Setup

   • Sign up for Codecov account
   • Connect GitHub repository
   • Generate repository upload token

2. GitHub Secrets Configuration

   • Add CODECOV_TOKEN to repository secrets
   • Configure token permissions

3. CI Workflow Updates

   • Add coverage generation to existing CI workflow
   • Configure coverage upload to Codecov
   • Set up coverage status checks

4. Coverage Configuration

   • Create codecov.yml configuration file
   • Define coverage targets and thresholds
   • Configure file exclusions and inclusions

5. README Integration

   • Add coverage badges showing current coverage percentage
   • Document coverage requirements for contributors

6. Branch Protection

   • Configure required status checks for coverage
   • Set minimum coverage requirements for merges

Dependencies

• Must be implemented after CI workflow is established
• Requires GitHub repository admin access for secrets setup
• Should be completed before implementing additional test suites

Definition of Done

• Coverage reports generated on every CI run
• Coverage badges visible in README
• Pull request coverage comments working
• Branch protection rules include coverage checks
• Coverage dashboard shows historical trends
• Team notified of coverage requirements

Files to Create/Modify

• • Add coverage steps
• • Coverage configuration
• • Add coverage badges
• Repository Settings - Add CODECOV_TOKEN secret

Notes

• Start with basic coverage setup, then enhance with advanced features
• Consider using Codecov's free tier for open source projects
• Coverage setup should not block development workflow
• Document coverage expectations in contributing guidelines