fix(embed): correct stale 'optional dependency' messaging

@huggingface/transformers is now a required dependency, so 'install the
optional dependency' guidance is wrong — and the real failure mode when the
embedder is unavailable is a missing local model, not a missing package.
Point setup/index-embeddings/model-integrity messages (and the maintenance
help + embedder/pooling comments) at the model: it ships with `apple-docs
setup`, or set APPLE_DOCS_ALLOW_REMOTE_MODELS=1 to fetch it.