docs: translate process docs to English; rename to English file names

Per the English-only repo policy, translate the remaining Portuguese process
docs to English and give them English file names:
- NeoReports-Decisoes.md -> DECISIONS.md (the ADR)
- plan.md -> PLAN.md
- CLAUDE.md and docs/MVP-Spec.md translated in place
- update references in CONTRIBUTING.md, CLAUDE.md, PLAN.md

Code, comments, XML docs and strings were already English; this covers the
process docs. Includes the prior CLAUDE.md rule additions from this branch
(English-only policy, PR-comment handling, autonomy table, per-task cycle).

Author: thiagoluga