shell_process_started / terminal_update events
from the cloud proxy, or MCP tool calls scoped to a shell. Clients use
the shellId to correlate terminal updates back to the shell they were
produced on (mirrors activeProcessByShell in the webapp).
This struct is designed to be used standalone on untyped tool calls
(via [Meta::merge_into]) rather than flattened into another variant’s
metadata, since shellId may appear on a wide variety of events.