Add all-mlops-github-actions extension

[ulises-jeremias]

Problem

The proposal places CI/CD GitHub Actions inside every MLOps template. That conflicts
with CPA/CNA architecture, where repository automation is an extension.

Embedding workflows in every template would duplicate YAML and make future CI policy
changes expensive.

Solution

Create an all-mlops-github-actions extension compatible with MLOps template types.

Scope:

• Compatible types:
  • mlops-sklearn
  • mlops-pytorch
  • mlops-tensorflow
• Add generated workflows under template/.github/workflows/:
  • ci.yml: ruff, type checks, pytest, optional Docker build smoke.
  • train.yml: manual/scheduled Continuous Training entrypoint.
  • model-gate.yml: compare metrics against configured baseline/registry.
  • deploy.yml: deploy only after explicit model promotion or manual trigger.
• Add template/docs/MLOPS_GITHUB_ACTIONS_GUIDE.md.
• Add template/docs/README.md.append.
• Add placeholder secrets documentation; never include real credentials.

Constraints:

• Use safe default permissions.
• Training/deploy jobs should be workflow_dispatch first; scheduled behavior must be
  explicit and documented.
• Do not assume GPU runners by default.
• Do not make CI fail because no remote MLflow server exists.

Acceptance criteria:

• The extension can be applied to the sklearn MLOps starter once it exists.
• Generated workflows are documented and can run in a minimal local/CI-compatible mode.

[ulises-jeremias]

Reopened after fully reverting #93 via #94. Work remains for the assigned contributor — please continue from this issue.