Local stub catalog under `fixtures/` or `examples/` for CI

[ulises-jeremias]

Summary

Minimal template+addon on disk for integration tests.

CNA mapping

CNA: local file:// testing against cna-templates checkout.

Acceptance criteria

• At least one template and one addon stub
• CI uses file:// URLs
• Documented in CONTRIBUTING

Blocked by

none

Blocks

none

Reference

Parity matrix (workspace): knowledge/create-python-app-parity-matrix.md
Reference repo: https://github.com/Create-Node-App/create-node-app

Parent epic

#6