setup-repo: public-repo hardening for write-perm workflows

[jonnyspicer]

Piloting in retina-tracker (#5, a PUBLIC repo), automated security review flagged HIGH on the workflow assets:

• claude.yml's @claude trigger is invokable by ANY commenter, now write-scoped — needs an author_association (OWNER/MEMBER/COLLABORATOR) guard on the if: gate. Applied in retina-tracker; propose a public-repo template variant.
• pull-requests: write on the review workflow's pull_request trigger is fork-exploitable on public repos; document pull_request_target + author allow-list guidance.
• SHA-pin the marketplace ref in settings.json and third-party actions (astral-sh/setup-uv@v5, anthropics/claude-code-action@v1); add top-level permissions: { contents: read } to ci.yml.
  Refs Pilot: adopt core:setup-repo in a Python repo #5.