Repo polish for OSS credibility (CONTRIBUTING, issue templates, demo)

[infralayers]

Goal

Right now the repo has code + a README but nothing that helps an outside contributor get oriented — this matters since the whole point of this project is to be a public credibility signal for InfraLayers.

Suggested scope (pick what's useful, don't over-build)

• CONTRIBUTING.md — how to build, test, and submit a change
• A GitHub issue template (bug report / feature request)
• A short demo GIF or screenshot in the README showing the popup actually enabling a proxy
• Double-check packages/example-extension/README.md and root README.md are accurate after issues Verify proxy auth + config switching end-to-end in Chrome #1-Firefox proxy-auth callback parity #3 land (e.g. remove the "Known gap" note once Firefox auth parity is done)

Definition of done

Your call — this is lower-priority polish, not a hard spec. Open a PR with whatever subset feels worth doing; no need to do all of it at once.

Suggested owner: whoever's free after the higher-priority issues (#1-#4) are done — good low-stakes task to split between you.