Errors and abort semantics (`CPA_*` codes)

[ulises-jeremias]

Summary

Typed errors for config parse, manifest load, PM fallback, abort, non-empty dir.

CNA mapping

CNA: errors.ts, NON_EMPTY_DIR_ERROR_CODE = CNA_NON_EMPTY_TARGET_DIR.

Acceptance criteria

• Error codes use CPA_ prefix
• Tests cover mapping/messages
• Non-empty target directory behavior matches CNA (--force override at CLI layer)

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