refactor: migrate from BunProc to Npm module

- Replace BunProc.install with Npm.add for plugin resolution

- Remove needsInstall check from installDependencies

- Update test files to use Npm module instead of BunProc

- Delete bun/index.ts and bun/registry.ts (no longer needed)

packages/opencode/src/plugin/shared.ts

@@ -1,7 +1,7 @@
 import path from "path"
 import { fileURLToPath, pathToFileURL } from "url"
 import semver from "semver"
-import { BunProc } from "@/bun"
+import { Npm } from "@/npm"
 import { Filesystem } from "@/util/filesystem"
 import { isRecord } from "@/util/record"
 
@@ -103,7 +103,7 @@ export async function checkPluginCompatibility(target: string, opencodeVersion:
 
 export async function resolvePluginTarget(spec: string, parsed = parsePluginSpecifier(spec)) {
   if (isPathPluginSpec(spec)) return resolvePathPluginTarget(spec)
-  return BunProc.install(parsed.pkg, parsed.version)
+  return Npm.add(parsed.version === "latest" ? parsed.pkg : `${parsed.pkg}@${parsed.version}`)
 }
 
 export async function readPluginPackage(target: string) {