This change makes system prompt text and tool changes part of the transcript rather than silently rewriting its starting conditions. This lets Pi record when instructions changed or tools became available, restore that state after resuming or navigating branches, and preserve cached prompt prefixes where the upstream supports it.
streamProxy dropped a final SSE event that was not newline-terminated
(the residual line buffer was never flushed after the read loop) and
called stream.end() without a result when no done/error event had been
seen, leaving EventStream.result() pending forever. Consumers like
agentLoop awaiting response.result() hung indefinitely when the server
closed the connection mid-response.
Flush the decoder and process the residual buffer after EOF, and push a
synthesized error event when the stream ended without a terminal event.
fixes#8996
