[P0] Integration tests: scaffold via catalog slugs (not only file:// URLs)

[ulises-jeremias]

Summary

Integration tests only scaffold with file:// URLs. Add coverage for catalog slug resolution end-to-end.

Acceptance criteria

• Test --template fastapi-starter with fixture catalog (CPA_CATALOG_FIXTURE=1, CPA_SKIP_GIT=1 or local file URLs in fixture)
• Test --addons github-setup slug resolution
• CI runs new tests on every PR

Depends on

• [P0] Resolve catalog slugs to template/extension URLs before scaffold #160

Reference

Maturity plan Tier 0 #2