shell_process_started / shell_process_completed
tool calls, on the terminal_update updates that append output to them,
and on shell_interpretation_failed. Together with [ShellIdMeta] it is
the explicit Shell tab ↔ process ↔ terminal association: the tool call id
alone is mode-dependent (the process id, a shell-map lookup, or a
per-event terminal), so clients rebuilding Shell state read this key
instead of inferring it from the id.
Like [ShellIdMeta], this struct is designed to be merged standalone into
any tool call’s _meta via [Meta::merge_into].