Why: the approved task-centric sidebar (proposal 2026-09-30, option A) needs
server-authoritative pin and archive state on conversations; client-side
storage would be fake state against the event-sourced model.
Evidence: go test ./... passes; omitempty keeps wire format and golden
samples byte-identical for zero values (backward compatible).
Impact: additive optional fields; consumers ignoring unknown fields are
unaffected (forward-compat principle). Backend consumes via submodule bump.
Refs: Agent-System-Documents docs/proposals/2026-09-30-ui-redesign-and-auth.md (P6/P8, R1)
weichen
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