Function persist_last_failure 

pub(crate) fn persist_last_failure(
    block_id: &str,
    failure: Option<&AgentFailure>,
    wstore: &Option<Arc<Store>>,
    event_bus: &Option<Arc<EventBus>>,
)

Persist or clear the last agent failure in block metadata.

Pass Some(failure) on a failed exit to write agent:last_failure into the block’s meta so the pane can recover the recovery banner on any future load without needing the ephemeral WPS event. Pass None on a clean exit to remove the key (setting it to JSON null triggers merge_meta’s delete path). Broadcasts a waveobj:update so active frontend subscribers see the change immediately via the block atom, not just on next full load.