[translation] Translate README to Portuguese (BR)

[mccarthy606]

The current README.pt-br.md is a stub. Replace it with a full translation of README.md.

What to do

1. Read the current English README.md
2. Replace README.pt-br.md content with a full Portuguese (Brazilian) translation
3. Keep the language nav row (only **Português (BR)** should be unlinked, the others linked)
4. Keep the same structure: Contents → What this is → The Stack → 7 products → Quick Start → What's Inside → The Operator Playbook → Why this exists → Acknowledgements → Status → License
5. Translate the table content but keep code blocks and command examples in their original form
6. Use natural BR-Portuguese, not literal translation

Reference

README.es.md and README.ru.md are full translations to crib structure from.

How to test

• Open the rendered file on GitHub — check that the language nav, badges, and tables render correctly
• Have a native speaker glance for tone

Out of scope

Do not translate stack/, workflows/, case-studies/, or other docs in this PR. Translation of those is a future, separate effort.

Effort: ~2-3 hours.