Adds a best-effort manifest v2 -> v3 converter, optional service-worker bundling via Rollup, a VS Code workspace file, and a short WORKSPACE_README.md explaining usage.

Files changed:

• scripts/convert-manifest.js (converter, --bundle-sw support, warnings)
• scripts/rollup.config.js (rollup config to bundle background scripts)
• package.json (bundle script + devDependencies)
• youtube.code-workspace (workspace settings)
• WORKSPACE_README.md (how to open workspace & run converter)

How to test locally:

1. Install dev deps (use legacy peer deps if npm complains):

npm install --legacy-peer-deps

2. Run the converter with bundling enabled:

node scripts/convert-manifest.js --input build/manifest2.json --output build/manifest3.json --bundle-sw

3. Verify outputs (not committed):

• build/manifest3.json — check manifest_version: 3, background.service_worker and host_permissions.
• build/generated_service_worker.js — review the generated bundle for APIs incompatible with MV3.

Notes & cautions:

• The converter is heuristic. Please review Content Security Policy, webRequest usage, and any APIs that require adaptation for Manifest V3 (e.g., use declarativeNetRequest for blocking network requests).
• The generated service worker is created by Rollup (or a fallback importScripts wrapper). Test thoroughly in Chrome/Chromium with the extension loaded unpacked.
• Build outputs were intentionally not committed. If maintainers want the bundle generated as part of CI, consider adding a CI job that runs npm ci && npm run bundle-sw and artifacts publishing.

CI workflow added:

• A GitHub Actions workflow .github/workflows/manifest-v3.yml was added in this branch. It runs on pull requests and feature/** pushes and performs:
  1. npm ci --legacy-peer-deps
  2. node scripts/convert-manifest.js --input build/manifest2.json --output build/manifest3.json --bundle-sw
  3. node scripts/validate-manifest3.js build/manifest3.json

This workflow bundles the service worker and validates the generated manifest3.json. It fails the check if manifest_version !== 3 or if background.service_worker is missing.

Suggested follow-ups:

• Add an explicit CI artifact upload if maintainers want the generated bundle attached to the job.
• Consider adding "type": "module" to package.json to avoid a Rollup ES module warning during action runs.

If you prefer, I can add the CI artifact step or adjust triggers to run only for PRs targeting master in the upstream repo.