build: migrate package management to pnpm and harden installs

[tharropoulos]

Change Summary

• switch the repo from npm to pnpm for local development and ci
• add pnpm-lock.yaml, remove package-lock.json, and declare pnpm as the
  package manager
• update package scripts and developer workflows to use pnpm
• update github actions to install dependencies with pnpm install --frozen-lockfile and run build, lint, typecheck, and test with pnpm
• use .nvmrc in ci and bump the node version from 22 to 24
• update the readme examples and development instructions to use pnpm
• remove dangling and unused dependencies from package.json
• add pnpm-workspace.yaml safeguards as a security change to reduce supply chain risk
• avoid issues from compromised packages and unexpected preinstall / postinstall scripts during dependency installation

PR Checklist

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