fix(events): address Codex review — balance step on step/start-listener throw, restore /goal guard

Codex review of PR-A found three blockers:

- A throwing step/start session-event listener left an unbalanced log
  (turn/start → step/start → turn/end with no step/end), which the invariants
  oracle rejects — masked because that rejection was itself contained as a
  throwing turn/end listener. Fix the root cause in the loop: mark the step open
  BEFORE appending step/start (Session.append pushes before notifying), so the
  outer catch's closeStep() appends the balancing step/end. The test now asserts
  the balanced outcome (stepEnd:1, step/end before turn/end); proven load-bearing
  (revert the reorder → the test goes red with stepEnd:0).
- Reintroduce the /goal-pattern guard deleted in the prior commit, migrated to a
  step/end session-event listener (the surviving step-boundary hook point), with
  a no-tools first step so it exercises the hasSteering continuation override.
- Update packages/core/agent/README.md: step boundaries are no longer agent/*
  emits.

packages/core/agent-loop/src/loop.ts

@@ -382,8 +382,13 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
       // turn-start listeners on the first step) joins before the request.
       drainSteering(ctx, agent, turn)
 
-      session.append('step/start', { turn, step })
+      // Mark the step open BEFORE the append: Session.append pushes the event
+      // to the log before notifying session/event listeners, so a THROWING
+      // step/start listener leaves step/start in the log. Setting stepOpen first
+      // means the outer catch's closeStep() then appends the balancing step/end
+      // (turn stays enclosed) instead of stranding an open step under turn/end.
       stepOpen = true
+      session.append('step/start', { turn, step })
 
       const abort = new AbortController()
       handle.setAbort(abort)