Add docs/how-to/install-minimal-and-add-skills.md covering the --minimal
/ --core-only / --profile=core install, the core/standard/full profiles,
and growing the surface live via /gsd:surface or on reinstall. Register
it in the docs/README.md How-to guides index.
Closes#832
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>