packages/opencode module shape

Never declare export namespace Foo. Use flat exports plus export * as Foo from "./foo"; modules under multi-sibling directories such as src/session and src/config each use their own file and self-export, not a barrel index.ts. Keep namespace-private helpers as non-exported top-level declarations in that file so export * as does not project them. Chose explicit ownership over barrels because hidden exports enlarge dependency surfaces.