cognition.ai/sidechains — client capability flag indicating that the
client renders sidechain threads and wants the proxy to deliver sidechain
rows (_meta["cognition.ai/sidechain"]) over session/update.
The agent only advertises [crate::AgentCapabilitiesMeta::sidechains] and
emits sidechain rows when the client sets this flag; clients that do not
opt in keep today’s behaviour (sidechain events stay hidden).