docs(dart): refresh morph_core / morph_storage package metadata for shipped parity

[cemal-yilmaz-bt]

Summary

Align package README and pubspec.yaml descriptions with current shipped Dart/TS parity (see docs/dart-parity.md).

Scope

• packages/dart/morph_core/README.md: remove stale MorphClient.init → UnimplementedError claim; document shipped runtime, pipeline, plugins.
• packages/dart/morph_core/pubspec.yaml: accurate description:.
• packages/dart/morph_storage/pubspec.yaml: clarify in-memory scope vs @morph/browser-storage.

Out of scope

• API/export refactors, logger typing changes, CI matrix.

[cemal-yilmaz-bt]

Merged via #34 (base branch f/plugin; closing keywords only fire on default branch).