Adds docs/manual-update.md with step-by-step procedure to install/update
GSD directly from source when npx is unavailable, including runtime flag
table and notes on what the installer preserves.
Adds a [!WARNING] notice at the top of README.md linking to the doc with
the one-liner install command.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
