chore: migrate repository workflows to pnpm

[okyrychenko-dev]

Summary

• Pin pnpm 11.21.0 as the repository package manager.
• Replace package-lock.json with an authoritative pnpm-lock.yaml.
• Preserve the approved esbuild installation script.
• Migrate composed package scripts, CI, and release automation from npm to pnpm.
• Configure CI to cache dependencies using the pnpm lockfile.
• Preserve the Node.js 22/24 matrix, coverage upload, release validation, and npm provenance.
• Update contributor and release documentation with the pnpm workflow.
• Keep consumer installation examples for npm, Yarn, and pnpm unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!