Improve GitHub visibility and contributor onboarding

[carrabre]

Summary

• Rework the README landing experience around a clear developer value proposition, featured projects, category navigation, reusable patterns, and a faster quick start.
• Add a custom 1280×640 social preview assembled from six live Playground experiences.
• Store all 39 experience social cards on the branch and use them from both the root catalog and every experience README instead of depending on CDN-hosted README images.
• Add a dependency-free pnpm play launcher and catalog validator covering all 39 experience manifests, READMEs, package scripts, source links, and live-demo links.
• Add contributor guidance, community standards, security reporting, issue forms, a PR template, generated-release categories, and lightweight GitHub Actions validation.
• Add a maintainer playbook for repository metadata, topics, release cadence, publishing checks, rights review, and weekly traffic measurement.

Verification

• npx -y node@20 scripts/validate-catalog.mjs
• npx -y node@20 scripts/play.mjs --list
• Verified 39 local WebP cards, 43 root README references, 39 experience README references, and zero remaining CDN card references
• pnpm install --filter ./experiences/vanguard-protocol --frozen-lockfile
• pnpm --filter ./experiences/vanguard-protocol build
• pnpm play vanguard-protocol (Vite launched successfully at 127.0.0.1:5188)
• Parsed all new YAML files successfully
• Rendered the revised README through GitHub's Markdown API
• git diff --check

GitHub admin follow-ups after merge

These settings cannot be applied by a pull request and are recorded in docs/maintainer-playbook.md:

1. Apply the proposed repository description and topics.
2. Upload .github/assets/social-preview.jpg under Settings → Social preview.
3. Create the new-experience, dependencies, and skip-changelog labels.
4. Enable Discussions only if a maintainer can respond consistently.
5. Publish the first grouped Playground release after the rights review described in the playbook.

[greptile-apps]

Greptile Summary

The PR substantially improves repository discovery and contributor onboarding.

• Reworks the root and experience READMEs around local social cards, navigation, reusable patterns, and quick-start instructions.
• Adds a catalog validator and interactive experience launcher.
• Adds contribution, security, issue, release, workflow, and maintainer guidance.

Confidence Score: 3/5

The PR should not merge until the validation workflow pins both executable actions to immutable commit SHAs.

The workflow still resolves actions/checkout and actions/setup-node through mutable v4 tags, so upstream tag movement can change the code executed for pull requests and main-branch validation without a repository change.

Files Needing Attention: .github/workflows/validate.yml

Important Files Changed

Filename	Overview
.github/workflows/validate.yml	Adds lightweight catalog validation, but its executable actions remain referenced through mutable major-version tags.
scripts/validate-catalog.mjs	Adds dependency-free validation for manifests, package scripts, README links, and local social-card signatures.
scripts/play.mjs	Adds catalog listing, interactive selection, and pnpm-based development-server launching.
README.md	Reorganizes the landing page around featured experiences, categories, reusable implementation patterns, and local setup.
CONTRIBUTING.md	Documents contribution scope, local setup, validation expectations, rights checks, and pull-request practices.

_{Reviews (2): Last reviewed commit: "Store all experience cards in the reposi..." | Re-trigger Greptile}

[greptile-apps]

Mutable workflow action references

The workflow resolves both executable actions through mutable v4 tags, allowing upstream tag movement to change the code run in pull-request and main-branch validation without a repository change. Pin both actions to full commit SHAs to make these CI dependencies immutable.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!