Split the streaming pipeline: API backends yield StreamEvents through
a channel, the runner reads them and routes to the appropriate UI pane.
- Add StreamEvent enum (Content, Reasoning, ToolCallDelta, etc.)
- API start_stream() spawns backend as a task, returns event receiver
- Runner loops over events, sends content to conversation pane but
suppresses <tool_call> XML with a buffered tail for partial tags
- OpenAI backend refactored to stream_events() — no more UI coupling
- Anthropic backend gets a wrapper that synthesizes events from the
existing stream() (TODO: native event streaming)
- chat_completion_stream() kept for subconscious agents, reimplemented
on top of the event stream
- Usage derives Clone
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
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