refactor(app): rename cached context accessors

[Brendonovich]

Summary

• rename cached server context access from createServerCtx to ensureServerCtx
• rename directory SDK and sync accessors to ensureDirSdkContext and ensureDirSyncContext
• retain create* naming for the private functions that always instantiate contexts

Stack

• stacked on fix(app): share synced session data anomalyco/opencode#33624

Testing

• bun typecheck in packages/app
• bun test --only-failures --preload ./happydom.ts ./src/context/server-sync.test.ts ./src/context/layout.test.ts ./src/context/layout-scroll.test.ts ./src/utils/refcount.test.ts
• repository-wide bun turbo typecheck push hook
• production first-navigation benchmark rerun was blocked because its configured port 3000 was already occupied; the parent baseline previously failed before collecting metrics because the smoke fixture did not render its expected session title

[Brendonovich]

Superseded by upstream PR anomalyco#33627 after the parent PR merged.