Refactor CI/CD to use a unified, reusable workflow architecture

[jdfalk]

Goal: Refactor all CI/CD workflows in ghcommon to use a single, reusable workflow template for build, test, lint, and deploy.

Acceptance Criteria:

• All major workflows (build, test, lint, deploy) are refactored to use a single reusable workflow file via 'workflow_call'.
• All jobs use consistent environment setup, caching, and matrix strategies.
• All secrets and environment variables are managed via repository/environment secrets.
• Documentation is updated to describe the new workflow structure and how to add new jobs.
• All legacy workflow files are removed or archived.

Technical Notes:

• Use GitHub Actions 'reusable workflows' and 'workflow_call' for DRY principles.
• Use matrix builds for OS, Python/Go versions, and dependency sets.
• Ensure compatibility with self-hosted and GitHub-hosted runners.
• Validate with dry-run and real PRs.
• See .github/workflows/reusable-unified-issue-management.yml for reference.