Add integration test framework

[HartBrook]

Summary

• Add YAML-driven integration test framework in internal/integration/
• Include test harness with filesystem isolation (t.TempDir())
• Add fixture-based tests for sync scenarios (basic, with personal, with languages, provenance order)
• Add live tests (behind live build tag) for real GitHub API validation
• Update Makefile with test-integration and test-live targets
• Document integration testing in CONTRIBUTING.md

Test plan

• Run make test-integration to verify mocked tests pass
• Run make test-live with GitHub auth to verify live tests
• Review fixture YAML files for completeness