ci: add GitHub Actions workflow and local CI tooling

- Add GitHub Actions workflow with 3 parallel jobs:
  * Main test job (vet, fmt, staticcheck, tests with race detector)
  * Per-package testing (db, onboarding, plugins/fossa)
  * Linting job (golangci-lint with 20+ rules)
- Add Makefile targets for local testing:
  * make test, test-verbose, test-coverage, test-race
  * make test-package PKG=<name> for focused testing
  * make ci-local to simulate full CI pipeline locally
  * make lint, fmt, vet for code quality checks
- Add .golangci.yml with comprehensive linter configuration
- Add 3 documentation guides:
  * CI_BEST_PRACTICES.md - comprehensive CI/CD patterns
  * CI_SETUP_SUMMARY.md - quick reference for setup
  * TESTING_GUIDE.md - developer guide for writing/running tests

CI runs on:
- Every push to main/master/develop branches
- Every pull request
- Manual trigger via GitHub UI

Performance:
- Total CI time: 2-3 minutes
- Tests complete in ~20ms per package
- Go module and build caching enabled
- No external API dependencies required

Author: RobertKielty