Minor docs update

[kathsherratt]

• Very minor edits to README: replace "covid19" with {topic}
• adds a CLAUDE.md to support context for future dev (written by claude, checked by kath)

[nicholasdavies]

Looks good to me but:
"key reviewers" in CLAUDE.md seems outdated -- Adam and John aren't much at LSHTM at the moment.
README.md -- would be good to explicitly list the "active" topics e.g. covid19 and (say) bvd-2026

[kathsherratt]

Thanks Nick! Good spot on the PR, that was a hangover from claude that I meant to remove.

My plan is to PR a few changes sequentially so was just opening this one to get started. Next up should be:

• Add a neutral landing page and move covid material into separate area of the website Archive repository of covid work #73
• Add a new section for BVD with appropriate tags etc to match
• Update the README / docs with appropriate instructions for adding new topics and content