Fix namespace reexport auto-imports

[mattiamanzati]

Summary

• distinguish namespace-valued reexports from ordinary top-level named reexports in namespaceImportPackages handling
• resolve namespace reexport targets through TypeScript module-specifier generation so package exports and editor preferences are preserved
• add policy and end-to-end completion/import-fix regression coverage

Behavior

Accepting the Effect completion now changes the generated import from:

import { Effect } from "effect"

to:

import * as Effect from "effect/Effect"

Testing

• pnpm setup-repo
• pnpm lint
• pnpm check
• pnpm test