Git download + disk cache (`~/.cache/cpa`) + refresh modes

[ulises-jeremias]

Summary

Clone/pull templates into cache with always/stale/manual refresh.

CNA mapping

CNA: git.ts + CNA_REFRESH / CNA_REFRESH_AFTER_HOURS / offline.

Acceptance criteria

• Default cache root ~/.cache/cpa
• Refresh modes: always | stale | manual
• Offline mode uses cache only
• Cache metadata read/write helpers
• Unit tests with fixtures (no network where possible)

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