[codex] Add acceptance fixture suite

[jooh]

Summary

Add a repo-owned manual acceptance fixture suite under acceptance/ for contributor-guided checks against the native CLI.

What Changed

• add acceptance/ with 5 curated scenarios:
  • image-lifecycle
  • dockerfile-build
  • template-node-mongo
  • local-feature
  • published-feature
• add acceptance/scenarios.json as a stable suite manifest for future automation
• add build/check-acceptance-fixtures.js to validate suite structure, manifest contents, and repo wiring
• document the suite in acceptance/README.md, README.md, and docs/architecture.md
• wire acceptance-fixtures-check into make tests

Why

The repo already has parity fixtures and smoke tests, but it did not have a stable set of small on-disk mock projects for manual verification. This adds a contributor-facing acceptance surface that is easy to inspect manually and structured so it can be automated later.

Impact

• contributors get a predictable manual validation suite for major devcontainer workflows
• one scenario explicitly exercises templates apply
• the suite stays separate from upstream/ and src/test/parity/, keeping repo-owned acceptance coverage distinct from compatibility fixtures

Validation

• make acceptance-fixtures-check
• make tests