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

[ulises-jeremias]

Summary

--template fastapi-starter and --addons github-setup are documented in README/CHANGELOG but fail at runtime because resolve_source() treats bare slugs as git URLs (git clone fastapi-starter).

CNA resolves slugs in options.ts (processNonInteractiveOptions) by looking up templates.json before calling core.

Acceptance criteria

• Add resolve_catalog_spec(spec) in CLI catalog module (templates + extensions)
• CLI resolves template, addons, and extend before create_python_app()
• Clear error when slug is unknown
• Works with CPA_CATALOG_FIXTURE=1 and live catalog

Reference

• CNA: packages/create-awesome-node-app/src/options.ts
• CPA gap: packages/create-python-app-core/src/create_python_app_core/paths.py (legacy slug)
• Maturity plan: knowledge/create-python-app-maturity-plan.md Tier 0 [Epic 0] CPA roadmap index — parity with create-node-app #1

Blocks

Interactive wizard, slug-based integration tests, README quick-start examples.