Replace yarn with npm@12

[risantos]

Description

Transition the project from using yarn to NPM v12 as package manager.

• Add devEngines.packageManager to package.json, which will also enable automatic dependency caching on CI via actions/setup-node.
• Update workflows and scripts accordingly to ensure compatibility and maintain functionality, now pinned to their commit hashes.
• Add a badge for CI test status in README.
• Add @fastify/pre-commit to allowScripts.
• Set the npm install-strategy to linked, as recommended on their docs.
  See https://docs.npmjs.com/cli/v12/commands/npm-install#install-strategy.

  We recommend that package authors use --install-strategy=linked during development to catch undeclared ("phantom") dependencies before publishing: the isolated layout only exposes a package's declared dependencies, so an import of a package that was never added to package.json can fail instead of resolving by accident and shipping broken. See Catching undeclared ("phantom") dependencies.

[Copilot]

Pull request overview

This PR transitions the repository’s tooling from Yarn to npm (targeting npm v12), updating CI workflows and project metadata to reflect the new package manager and adding a CI status badge to the README.

Changes:

• Remove yarn.lock and update GitHub Actions workflows to use npm ci / npm run … instead of yarn …, while pinning actions to commit SHAs.
• Add devEngines.packageManager (npm v12) and an allowScripts entry to package.json.
• Add a CI tests status badge to README.md.

Reviewed changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
yarn.lock	Removed Yarn lockfile as part of the migration away from Yarn.
package.json	Adds npm v12 package manager metadata and allowScripts configuration.
.github/workflows/tests.yaml	Switch CI install/lint/test steps from Yarn to npm and pin GitHub Actions by SHA.
.github/workflows/release.yaml	Switch release workflow dependency installation from Yarn to npm and pin GitHub Actions by SHA.
README.md	Adds a CI test status badge.

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