fix(agent-loop): surface throwing step-end listener as turn error via failTurn

closeStep() previously caught and silently swallowed a throw from
agent/step-end emit. In the normal no-tool/no-steering path, this
caused runTurn to reach closeTurn(true) with reason still
{kind:completed}, so the session recorded a completed turn with
zero error events — even though a plugin had failed at a loop
boundary. This violates the contract that a throwing plugin is
contained as a turn error, not a silent success.

Now the catch calls failTurn(toError(error)), which appends the
single error event and sets reason={kind:error,…}. failTurn is
idempotent (errorReported guard), so existing error paths that
call failTurn after closeStep are unaffected.

Add regression test: a throwing agent/step-end listener during a
successful step now produces exactly one error event, a turn/end
with reason error, balanced boundaries (step/end before turn/end),
and a surviving loop.

packages/agent-loop/src/loop.ts

@@ -180,9 +180,12 @@ async function runTurn(ctx: Context, agent: LoopAgent, handle: LoopHandle, turn:
     session.append('step/end', { turn, step })
     try {
       ctx.emit('agent/step-end', agent, turn, step)
-    } catch {
-      // contained: step/end is already recorded, so balance holds; a throwing
-      // step-end listener is the listener's bug, not the loop's.
+    } catch (error: unknown) {
+      // step/end is already recorded so balance holds; surface the throwing
+      // listener as a turn error via failTurn (idempotent). This prevents a
+      // throwing step-end listener from producing a silent "completed" turn
+      // when the step itself succeeded (the normal-path closeStep call).
+      failTurn(toError(error))
     }
   }