chore: curate OSS presentation — badges, CI, templates, contributing

[yesil]

Summary

Curate the project so it reads as mature and trustworthy to a first-time visitor on GitHub and npm, and add the missing community/automation scaffolding.

• README: shields.io badges (npm version, downloads, minzipped size, zero-deps, license, CI), explicit npm link, new Links + License sections, Contributing now points to CONTRIBUTING.md
• CI: GitHub Actions workflow (Node 22, npm ci → npm test → npm run build) on push to main and all PRs
• Templates: YAML issue forms (bug/feature), issue config, PR template
• CONTRIBUTING.md: dev setup, browser-test model, project structure, Conventional Commits
• examples/index.html: restyled dependency-free demo landing (was a bare list)
• package.json: sideEffects: false, engines: node >=22, lint script
• eslint.config.mjs: ignore dist/** and bundled swc.js

Type of change

• Documentation
• Other — repo presentation / CI / tooling

Test plan

• npm test passes (37/37) on Node 22
• npm run build reproduces committed artifacts (no churn)
• CI workflow YAML validated with @action-validator/cli
• GitHub repo About (description, homepage, topics) updated via gh

Reviewer notes

• Lint is intentionally not a CI gate. The repo is not prettier-clean — there is pre-existing formatting debt in src/router.js, src/makeLitObserver.js, and 3 test files. CI runs tests + build only (both green). npm run lint is available locally; npm run lint -- --fix will clean the debt whenever you want to re-enable a lint gate.
• The CI badge will show "no status" until the workflow runs for the first time on main after merge.
• Out of scope (flagged, not changed): exports["./router"] points to ./src/router.js while the main entry uses dist/ — a real packaging inconsistency worth a separate fix.

Co-Authored-By: Claude Opus 4.7 noreply@anthropic.com