Test dev merge commits and document the dev-to-master release workflow

[coder13]

Problem

Pull requests are checked, but .github/workflows/ci.yml only runs on pushes to master. The merge commit created on dev is therefore not automatically revalidated. CONTRIBUTING.md also still documents npm, the pre-workspace layout, and older test tooling.

Acceptance criteria

• Run the full CI workflow on pushes to dev as well as master.
• Document the intended branch flow: feature branch → dev → release-candidate validation → reviewed promotion to master.
• Update contributing setup for Node 22, Yarn classic workspaces, Vite, Docker-backed MongoDB/PostgreSQL/Redis, root Turbo commands, Jest, and Cypress.
• Document required promotion evidence and who/what may block a release.
• Add or document a lightweight release checklist that links deployment and rollback verification.
• Remove stale npm/CRA/Enzyme instructions without unrelated documentation churn.