Add linting infrastructure and test separation for CI readiness

[dzyanis]

Summary

This PR prepares the repository for CI/CD integration by adding linting infrastructure, a Makefile for common tasks, and separating unit tests from integration tests.

Changes

Linting Infrastructure

• Added golangci-lint configuration with default settings
• Fixed all existing linter warnings

Build Automation

• Added Makefile with common development tasks:
  • make lint - run golangci-lint
  • make test - run unit tests only (short tests)
  • make test-integration - run integration tests

Test Organization

• Separated unit tests (short, no external dependencies) from integration tests
• Unit tests can be run with go test -short ./...
• Integration tests require API credentials and are skipped with -short flag
• This separation enables faster CI feedback loops

Benefits

• CI Ready: All components needed for automated testing and linting in CI
• Code Quality: Consistent code style enforced by linter
• Developer Experience: Easy-to-use Makefile commands for local development
• Faster Feedback: Separated tests allow quick unit test runs during development

Running

• All unit tests pass
• golangci-lint runs clean with no warnings
• Some integration tests are currently broken and should be fixed

Next Steps

This PR sets the foundation for:

• Fix all integration tests
• GitHub Actions CI workflow
• Expand the number of included linters