Make marginal easier to find: Marketplace listing, social preview, README demo, Discussions #210

Context

marginal and marginal-action work, and the README now shows benchmark results (#179), but someone looking for a self-hosted PR reviewer is unlikely to find them. Both repos have no homepage link, no custom social preview (shared links show GitHub's default card), and no place to ask questions other than issues.

This issue covers what can be done inside the repos so that people who search, browse the Marketplace, or follow a shared link land on something clear. It's a checklist; each item can be its own small PR, and contributors are welcome to pick one.

Tasks

Listing and metadata

• Publish marginal-action on the GitHub Marketplace (its action.yml already has branding), and link the listing from marginal's README.
• Set the repo homepage on both repos (PyPI page for marginal, Marketplace listing for marginal-action).
• Custom social preview image for both repos (1280×640, name + one-line pitch), so shared links render a real card.
• Review topics on both repos; add any that people actually search for (e.g. code-quality, ai-agents, pr-review).

README

• A short GIF or screenshot near the top: a PR with an inline finding, its quoted evidence, and the verdict line.
• PyPI version badge and a link to the review-quality results.
• A "See it live" link to real reviewed PRs on exactml/marginal-client.

Community

• Turn on GitHub Discussions (Q&A, Show and tell), and point "questions" there from the issue template chooser (.github/ISSUE_TEMPLATE/config.yml).
• Submit marginal to relevant curated awesome-* lists for code review, GitHub Actions and LLM developer tools, following each list's own contribution rules.

Out of scope

Posting announcements on external sites; maintainers handle that themselves.

Acceptance criteria

• marginal-action is installable from the Marketplace.
• A link to either repo shared in chat shows the custom preview card.
• The README shows marginal in action before the install section.