flush the pending queue before returning

tokio's File buffers; an append that returned Ok could still be in memory
when the next take() read the file. Caught by multiple_appends_accumulate on
a runner, never on a developer machine.

src/core/nervous/pending.rs

@@ -66,6 +66,11 @@ pub async fn append(agent_data_dir: &Path, items: &[PendingSurfacing]) -> anyhow
         buf.push('\n');
     }
     file.write_all(buf.as_bytes()).await?;
+    // tokio's File buffers. Without this the bytes can still be in memory when
+    // the next take() reads the file, and an append that returned Ok is simply
+    // absent -- which is what multiple_appends_accumulate caught on a runner
+    // and never caught on a developer machine.
+    file.flush().await?;
     Ok(())
 }