[translation] Translate README to Japanese

[mccarthy606]

The current README.ja.md is a stub. Replace it with a full Japanese translation of README.md.

What to do

1. Read the current English README.md
2. Replace README.ja.md with a full Japanese translation
3. Keep the language nav row (only **日本語** unlinked, others linked)
4. Same structure as English: Contents → What this is → The Stack → 7 products → Quick Start → What's Inside → The Operator Playbook → Why this exists → Acknowledgements → Status → License
5. Keep code blocks and CLI commands in original form

Reference

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

Effort: ~2-3 hours.