Set up contribution infrastructure (CONTRIBUTING, CI, templates, commitlint)

[AlexMost]

Context

The preversion hook (lint && test && lib:build) runs only locally. Nothing is enforced on CI. CONTRIBUTING.md is missing. Depends on a separate ecosystem-level decision on release & commit conventions.

ttag-specific notes (vs ttag-cli):

• Tests use Mocha + ts-node (not Jest)
• Build produces three artifacts (CJS, ESM, .d.ts) via lib:build (babel + webpack + tsc)
• Has a separate dtslint step for public types
• Documentation lives in website/ (Docusaurus) — separate workflow for deploy, but only after the Docusaurus v3 migration

Scope

• CONTRIBUTING.md:
  • Dev setup (Node version from engines)
  • Tests (npm test, npm run test:file), lint, prettier, dtslint
  • How to run the website locally (npm run doc:install, npm run doc:serve)
  • Commit conventions
  • PR checklist (tests, types, dtslint, prettier-check)
• PR template
• Issue templates (bug, feature, types-related)
• CI workflow: install + lint + test + lib:build + dtslint on a Node matrix
• Release workflow per ecosystem decision
• Deploy workflow for the website (gh-pages) — replace manual doc:publish with GitHub Actions. Done as part of the Docusaurus v3 migration issue — here we just record that CONTRIBUTING.md describes the new deploy flow, not legacy doc:publish
• commitlint + husky

Out of scope

• CODEOWNERS / branch protection — separate task
• API-extractor / publish API tracking — separate discussion

Definition of Done

• CONTRIBUTING.md and templates in .github/
• CI workflow green
• Release workflow set up or consciously deferred
• commitlint blocks invalid commits locally
• Website deploy via GH Actions, no manual doc:publish (or documented why it stays manual)