Paths resolution (GitHub URL, file://, slugs, ?ref=)

[ulises-jeremias]

Summary

Resolve template/extension locations including cache scoping and strict repro.

CNA mapping

CNA: paths.ts + CNA_STRICT_REPRO / CNA_CACHE_DIR.

Acceptance criteria

• Supports GitHub URLs, file://, legacy slugs
• Honors CPA_CACHE_DIR and CPA_STRICT_REPRO
• Unit tests for each scheme

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

#4