When account.blockStreaming is unset, pass undefined instead of false for
disableBlockStreaming so downstream get-reply-directives inherits the
agent-level default rather than forcing block streaming on.
Affected paths: button-click interactions (handleInteractiveMenuInteraction)
and model picker confirmations (handleModelPickerInteraction).
slash-http.ts already used undefined correctly; this brings monitor.ts
into alignment.
Addresses Codex P2 review: 'Preserve inherited block-streaming default
for model picker replies' and 'Preserve default block-streaming behavior
for button replies'.
ec10de7361