`cpa.config.json` schema + custom options

[ulises-jeremias]

Summary

Template config schema analogous to cna.config.json with custom prompts/options.

CNA mapping

CNA: config.ts (CnaConfig, CnaCustomOption).

Acceptance criteria

• Documented JSON schema for cpa.config.json
• Custom options feed --set / interactive prompts
• Parse errors raise CPA_CONFIG_PARSE

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