DO NOT MERGE before Aug 26 org rename

[Abdelsalam-Abbas]

The GitHub org diff-use is renamed to prism-science on Wed Aug 26 (~4pm PT). This repo pins the old org name in site config and in the giscus comment widget.

Changes:

• overrides/partials/comments.html: giscus data-repo changed to prism-science/logbook. This one is breaking after the rename: giscus resolves the repo via the GitHub API, which does not follow org-rename redirects, so new comments would fail with the old value. data-repo-id and data-category-id are internal IDs and survive the rename unchanged.
• mkdocs.yml: repo_url updated (web link, redirect-safe, but stale otherwise).
• docs/.authors.yml: avatar URL org segment updated to prism-science/diff-use.github.io.

Decisions:

• Merge this after the rename lands on Aug 26, per the title. Merging before would break giscus against the still-current diff-use name.
• The diff-use.github.io repo NAME in the avatar URL is kept: an org rename does not rename the Pages repo itself. When that repo is renamed to prism-science.github.io, update this URL again.
• diffuse.science URLs and "Diffuse Project" prose untouched, different name, no hyphen.

Out of repo:

• The giscus GitHub App must be installed on the prism-science org (installations usually carry over on rename, but verify) and Discussions must stay enabled on the repo.
• Rename diff-use.github.io to prism-science.github.io separately if Pages should keep working at the org domain.

Validation:

• yaml.safe_load passes on mkdocs.yml and docs/.authors.yml.
• git grep diff-use shows only the intentionally kept diff-use.github.io repo name.