repo-settings: configure repository social preview

[hummbl-dev]

Summary\nSet the BIF repository social preview image for GitHub and link unfurls.\n\n## Acceptance criteria\n1. Use docs-site/assets/bif-social-preview.svg or an exported PNG equivalent as the source.\n2. Configure the repo social preview in GitHub settings.\n3. Confirm preview renders correctly on GitHub and social/link unfurls.\n\n## Blocker\nGitHub social preview configuration is a repo settings action, not a file-only change.

[hummbl-dev]

Status: the social preview asset exists at docs-site/assets/bif-social-preview.svg, but GitHub still requires a manual Settings > Social preview upload to attach it. That part is not exposed through the repo API/gh CLI.