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

[AlexMost]

Context

Currently the preversion hook (build && test) runs only locally — nothing is enforced on CI, and external PRs are not validated automatically. CONTRIBUTING.md is missing, so newcomers have to reverse-engineer how to run tests and what commit style to use.

This depends on a separate decision on release & commit conventions for the ttag ecosystem (Conventional Commits + tool choice between release-please and changesets).

Scope

• CONTRIBUTING.md at the repo root:
  • Dev setup (npm install, Node version from engines)
  • How to run tests, lint, build
  • Commit message format (Conventional Commits)
  • PR checklist (tests, lint, npm run readme after command changes)
  • Release flow
  • Code of Conduct (standard Contributor Covenant)
• .github/PULL_REQUEST_TEMPLATE.md — minimal (description, related issue, checklist)
• .github/ISSUE_TEMPLATE/ — separate templates for bug / feature / question. Bug requires: version, minimal repro, expected vs actual behavior
• .github/workflows/ci.yml — on push/PR: install + lint + test + build. Node version matrix matching engines
• .github/workflows/release.yml — release-please or changesets (per ecosystem decision)
• commitlint + husky commit-msg hook for locally-enforced Conventional Commits
• If we go with release-please/changesets, adjust preversion/prepublish flow to avoid duplication with CI

Out of scope

• Code of Conduct enforcement — standard Contributor Covenant copy-paste is enough for a personal-org project
• CODEOWNERS / branch protection rules — separate task

Definition of Done

• CONTRIBUTING.md merged and linked from README
• PR/issue templates show up when creating new PRs/issues
• CI workflow green on main and on a test PR
• Release workflow set up and tested (or consciously deferred and documented why)
• commitlint blocks invalid commits locally
• Content aligned with the parallel work in ttag and msgai repos