fix: support scoped npm plugins

[riatzukiza]

Scoped Plugin Bug

• Loader currently does plugin.split("@") (packages/opencode/src/plugin/index.ts:37), so a scoped name like @promethean-os/opencode-openai-codex-auth becomes pkg === "" and version === "promethean-os/opencode-openai-codex-auth".
• BunProc.install then records that empty package key in the cache manifest (packages/opencode/src/bun/index.ts:60-94), producing "": "promethean-os/opencode-openai-codex-auth" and never installing the real module. The follow-up import resolves to /home/err/.cache/opencode/node_modules, triggering the “Cannot find module …” error you’re seeing.
• Parse plugin strings from the rightmost @ (or use npm-package-arg) so scoped packages stay intact, e.g. const at = plugin.lastIndexOf("@"); const pkg = at > 0 ? plugin.slice(0, at) : plugin; const version = at > 0 ? plugin.slice(at + 1) : "latest";.
• After patching, clear ~/.cache/opencode and restart so the dependency file is regenerated with the correct scoped package key.
  Next step: adjust the loader parsing logic and rebuild; rerun with the scoped plugin to confirm it resolves.

closes: #3392

[riatzukiza]

I added these because my bot would have dumped a load of memory files you may not want, and this is a common/popular agent framework that people should be able to/allowed to use on this project with out polluting the git.