docs: actually apply the English translation (content, not just renames)

The previous commit on this branch carried the renames (DECISIONS.md, PLAN.md)
but the file *contents* were still Portuguese — the translated text was in the
working tree but had not been staged into that commit. This commit applies the
English content for CLAUDE.md, DECISIONS.md, PLAN.md, docs/MVP-Spec.md and the
CONTRIBUTING.md references.

Author: thiagoluga