agent: forward unhandled turn failures to manager

run_claude now keeps a 20-line stderr ring buffer and bails with it
inline (was just 'exit <status>'). agent serve loop, on Failed (not
PromptTooLong — that's already absorbed by drive_turn's compaction
retry), sends the error body to manager via the normal hyperhive
send. swallows transport errors — failure is already in journald
and the events sqlite. manager-only harness (hive-m1nd) is unchanged
so it doesn't try to notify itself.

hive-ag3nt/src/turn.rs

@@ -3,6 +3,7 @@
 //! manager surface) and their wake-prompt wording; the spawn shape,
 //! arg-vector, stdin plumbing, and stream-json pumping are identical.
 
+use std::collections::VecDeque;
 use std::path::{Path, PathBuf};
 use std::process::Stdio;
 use std::sync::atomic::{AtomicBool, Ordering};
@@ -281,6 +282,13 @@ async fn run_claude(prompt: &str, files: &TurnFiles, bus: &Bus) -> Result<bool>
             }
         }
     });
+    // Keep the last STDERR_TAIL_LINES of stderr so a non-zero exit can
+    // include real context in the bail message (and downstream in the
+    // failure notification to the manager) instead of just "exit 1".
+    const STDERR_TAIL_LINES: usize = 20;
+    let stderr_tail: Arc<Mutex<VecDeque<String>>> =
+        Arc::new(Mutex::new(VecDeque::with_capacity(STDERR_TAIL_LINES)));
+    let tail_clone = stderr_tail.clone();
     let pump_stderr = tokio::spawn(async move {
         let mut reader = BufReader::new(stderr).lines();
         while let Ok(Some(line)) = reader.next_line().await {
@@ -293,6 +301,11 @@ async fn run_claude(prompt: &str, files: &TurnFiles, bus: &Bus) -> Result<bool>
             // surfaces when claude exits non-zero.
             tracing::warn!(line = %line, "claude stderr");
             bus_err.emit(LiveEvent::Note(format!("stderr: {line}")));
+            let mut t = tail_clone.lock().unwrap();
+            if t.len() >= STDERR_TAIL_LINES {
+                t.pop_front();
+            }
+            t.push_back(line);
         }
     });
 
@@ -301,7 +314,12 @@ async fn run_claude(prompt: &str, files: &TurnFiles, bus: &Bus) -> Result<bool>
     let _ = pump_stderr.await;
     let too_long = prompt_too_long.load(Ordering::Relaxed);
     if !status.success() && !too_long {
-        bail!("claude exited {status}");
+        let tail = stderr_tail.lock().unwrap();
+        if tail.is_empty() {
+            bail!("claude exited {status} (no stderr)");
+        }
+        let tail_str = tail.iter().cloned().collect::<Vec<_>>().join("\n");
+        bail!("claude exited {status}\nstderr tail:\n{tail_str}");
     }
     Ok(too_long)
 }