docs: tighten public copy, add comparison, templates, and social preview

* docs: tighten README, CONTRIBUTING, and CLA prose

* docs: add platform comparison and README FAQ

* chore: add issue and pull request templates

* chore: add repository social preview image

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,28 @@
+<!-- Thanks for contributing. Keep one logical change per pull request. -->
+
+## What this changes
+
+<!-- A short summary of the change and the problem it solves. Link the issue it addresses. -->
+
+Closes #
+
+## Type of change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Refactor or cleanup
+- [ ] Documentation
+- [ ] Other
+
+## Checklist
+
+- [ ] `pnpm build` succeeds (shared types, server, and web all build)
+- [ ] `pnpm dev` starts the server and web client without errors
+- [ ] Tests pass where applicable (`pnpm test`)
+- [ ] I updated the relevant `docs/systems/` spec if this changed schema, API routes, WebSocket events, the federation protocol, permissions, or the design system
+- [ ] This change resolves the correct federated identity where it compares IDs, checks permissions, or talks to remote servers (no assumption of a single global user ID)
+- [ ] I have read and agree to the [CLA](../CLA.md)
+
+## Notes for reviewers
+
+<!-- Anything that helps review: screenshots for UI, migration notes, edge cases, follow-ups. -->