Add Node build pipeline to package userscripts as Chromium (blink) and Firefox (moz) extensions

[digitalby]

Motivation

• Provide a reproducible, CI-friendly way to ship the existing *.user.js sources as browser extensions while keeping the original userscript files in place for existing raw GitHub installs.
• Support two immediate targets (blink/ for Chromium and moz/ for Firefox) and make future platform outputs (e.g. Safari) easier to add.

Description

• Add a Node build script at tools/build-extensions.mjs that scans root *.user.js files, parses userscript metadata, inlines @require files that reference the repo's raw GitHub URLs, and produces bundled content scripts.
• Add package.json scripts (build:extensions, clean:extensions) to run the build locally or in automation.
• Add a GitHub Actions workflow at .github/workflows/build-extensions.yml that runs the build, zips blink/ and moz/ outputs, and uploads them as artifacts.
• Emit extension outputs in blink/ and moz/ (each contains scripts/ and a generated manifest.json), and document the build/load steps in README.md; original .user.js sources are not moved or renamed.

Testing

• Ran npm run build:extensions which completed successfully and reported the number of bundled userscripts.
• Verified generated manifests by inspecting blink/manifest.json and moz/manifest.json to confirm expected content_scripts, run_at mappings, and Firefox browser_specific_settings.
• Confirmed the build created per-target scripts/*.js bundles in both blink/ and moz/ directories.

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Codex Task