fix: support the opencode2 beta-18743+ plugin loader

The v2 loader rejects file-path plugin config entries ("configured
plugin path must be a directory"), so the documented dist/server.js
registration no longer activates the plugin. build:v2 now emits the
real bundle at dist/server/index.js (the directory entrypoint) and the
exports map resolves ./server directly to it.

verify-release-artifact now requires dist/server/index.js, imports the
installed ./server subpath from a clean temp install, and parses the
object-form npm pack --json output emitted by npm >= 12.