Template/extension loaders + merge model

[ulises-jeremias]

Summary

Load files from template + layered addons/extensions with merge semantics.

CNA mapping

CNA: loaders.ts, package.ts.

Acceptance criteria

• Layer template → addons → extend
• Merge pyproject/tooling manifests appropriately for Python
• Tests for conflict/override behavior

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