fix: scope Npm.add() lock to per-package key

Use npm-install:${pkg} instead of a global npm-install lock so
concurrent installs of different packages can run in parallel.

packages/opencode/src/npm/index.ts

@@ -56,7 +56,7 @@ export namespace Npm {
   }
 
   export async function add(pkg: string) {
-    using _ = await Lock.write("npm-install")
+    using _ = await Lock.write(`npm-install:${pkg}`)
     log.info("installing package", {
       pkg,
     })