chore: ignore foreign lockfiles and add packageManager configuration

[sonusharma6-dsa]

Fixes #45

Summary

This PR addresses lockfile hygiene by preventing non-npm lockfiles from cluttering working trees and enforcing npm as the package manager across the repository:

• Added foreign lockfiles (bun.lock, bun.lockb, pnpm-lock.yaml, yarn.lock) to .gitignore.
• Added "packageManager": "npm@10.8.2" in root package.json and demo/package.json to enable Corepack enforcement.
• Updated CONTRIBUTING.md to explicitly specify npm as the required package manager.

Testing

• Verified all tests pass (48/48 tests passing).
• Verified npm run lint and npm run typecheck pass cleanly.

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[sonusharma6-dsa]

Closing this PR as maintainers indicated on #53 that the repo should not be made npm-only or ignore pnpm-lock.yaml. Thanks!