fix(sdk): scan not load the workspace for library siblings, add build integration tests

resolveLibrarySibling used loadWorkspace, which throws on ANY workspace-wide shape
problem -- a typo in one unrelated plugin's package.json broke `s2s build
plugins/healthy-plugin` for every other healthy plugin declaring a library.
Switch to scanWorkspace (collects problems) with the same "unread member" warning
workspace/siblings.ts already applies to sibling contract resolution on this same
single-target build path.

Also add end-to-end coverage through the real buildPlugin()/esbuild.build() for both
resolution sources -- a unit test of the resolver alone can't catch a regression in
the nodePaths/alias/mainFields/typecheck-paths wiring.