Python version pin (`.python-version` + requires-python)

[ulises-jeremias]

Summary

Pin supported Python (recommend >=3.12) via .python-version and workspace intersection.

CNA mapping

CNA: .node-version + engines.node.

Acceptance criteria

• .python-version committed
• requires-python consistent across members
• CI matrix documents the pin

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

#2