docs/gbrain-sync.md: setup walkthrough, privacy modes, cross-machine
workflow, secret protection, two-machine conflict handling, uninstall,
troubleshooting reference.
docs/gbrain-sync-errors.md: problem/cause/fix index for every
user-visible error. Patterned on Rust's error docs + Stripe's API
error reference.
README.md: 'Cross-machine memory with GBrain sync' section near the
top (discovery moment), plus docs-table entry.
Garry Tan