fix: skip native build when a bundled prebuild matches the platform

[AlexanderSlaa]

Summary

• npm's automatic node-gyp invocation (via binding.gyp + gypfile) ran unconditionally on every install, even on platforms where bin/<triplet>/ already had a matching prebuild — forcing every consumer on a supported architecture to have a full C++ toolchain, pkg-config, libmnl-dev, and libsodium-dev installed just to immediately discard the build and use the prebuild at runtime anyway.
• gypfile is now false, replaced with an explicit install script (scripts/install.js) that checks for a matching prebuild first and only falls back to node-gyp rebuild when none exists.
• node-gyp moved from devDependencies to dependencies, since the fallback build path needs it available for consumers too, not just local development.
• Unsupported architectures and local development (no bin/ prebuild present yet) are unaffected — they still build from source exactly as before.

Test plan

• npm install in a fresh clone (no bin/ prebuild) still builds from source successfully and all existing tests pass (npm test, including the real kernel-backed netlink tests, run as root)
• npm run package && npm pack, then installed the resulting tarball as a dependency in a separate test project — confirmed via --foreground-scripts that the install step prints "using the bundled prebuild... skipping the native build" and never invokes node-gyp
• Required the installed module and confirmed generatePrivateKey()/publicKey()/WireGuardClient all work correctly from the prebuild
• Reviewed .github/workflows/ci.yml — no changes needed; the test and package-dry-run jobs run before any prebuild exists (so they still exercise the from-source build path with the toolchain they already install), and release builds+stages its own x86_64 prebuild as before