Plugin hooks can now stream results as async generators

What

• Hooks can now be implemented as async generator functions (async function*) that yield a stream of chunks instead of returning one final value. Claude Code validates this: hooks registered on events designated as "streaming" events must be async generators, and hooks on non-streaming events must not be.
• A full streaming dispatch pipeline was added end-to-end: the plugin host gained dispatchStream/linkStreams/withinSteps alongside the existing dispatch/link/within, the hooks-worker protocol gained matching frame types (dispatch_stream, stream_pull, stream_step, next_pull, next_close, stream_close), and the sandboxed tool/environment worker gained its own dispatch_stream/stream_pull/stream_close messages backed by a streams map, so a dispatch can yield incremental results.
• Currently only one event, turn.step, is enabled to use the streaming/generator form. It reconstructs each model request/response as discrete stream chunks and lets a registered plugin hook change which model a request uses. If a hook proposes a model that policy doesn't allow, Claude Code keeps the original model and logs a warning plus a hook-effect telemetry event.

Why This lays the groundwork for hooks that need to see or influence a request incrementally rather than only before or after it completes; today that's used to let a plugin hook override or veto the model used for a turn, subject to policy.