Rewrite streamManager/index.ts to export the full consumed surface (all
listeners, startAgentSessionRun, PersistenceBackend contract, TranslationBackend,
stream types); route all 15 external deep imports through it. Remove the
internal-only context/ and lifecycle/ sub-barrels so internal siblings import
directly. Expose AiStreamRequest/CallOverrides on the ai/types barrel and route
ChannelAdapter through the channels barrel.
Two deep imports remain, blocked by nested sub-barrels in sibling dirs
(runtime/claudeCode, channels/security) — deferred to those dirs' cleanup.