[Tracking] docs(readme): add prerequisites section and install verification step (was PR #3781 from fork)

> This is a tracking issue. The original PR [gsd-build/get-shit-done#3781](https://github.com/gsd-build/get-shit-done/pull/3781) was opened by @okwn from fork `okwn:contrib/get-shit-done/readme-troubleshooting` against the previous repo at `gsd-build/get-shit-done`. During the migration to `GSD-redux/get-shit-done-redux`, this PR could not be auto-recreated because the head branch lives in a contributor fork and the migration tooling cannot push to fork branches. **For the original author (@okwn)**: when you're ready, please re-base your branch against `GSD-redux/get-shit-done-redux` and open a fresh PR. Link this issue in the PR body so we can close it. --- ### Original PR title docs(readme): add prerequisites section and install verification step ### Original PR body Add a Prerequisites subsection under Getting Started listing Node.js ≥18, npm ≥9, and Git as requirements, with version-check commands. Add a Verify your install subsection with gsd-tools --help to confirm commands are available after install. Move the skip-permissions note to the right context (after the install command, not duplicated before the install-only section).