fix(agent-loop): decide turn balance + idle-injection flush from the log (review #32)

Session.append pushes the event BEFORE notifying session/event listeners,
so a throwing listener leaves the event in the log while the line after
the append (a boolean flag) never runs. Both turn-balance decisions were
gated on such flags, so a throwing listener could strand an open turn or
skip a durability checkpoint.

- loop.ts: the outer catch decided "turn/end owed" from `turnStarted`.
  A throwing listener on the turn/start append left turn/start logged but
  the flag false → catch rethrew and skipped turn/end → permanently open
  turn (violating ADR 0017). Now decided from the log (this turn's
  turn/start present), so the turn is always balanced; only a genuine
  pre-push failure (non-serializable trigger — turn/start never logged) is
  rethrown to the runLoop backstop. Removed the now-dead `turnStarted`.

- agent.ts inject(): the idle one-shot-turn flush was gated on a
  `turnRecorded` flag set after append('turn/end'); a throwing turn/end
  listener skipped the flush, losing the balanced in-memory injection turn
  on crash. Now the flush decision is read from the log, the synthetic
  turn/end append contains a throwing listener (turn stays balanced), and
  a failing idle flush is reported via agent/error (step 0 convention) AND
  the logger — mirroring the loop's post-turn/end flush path — with a
  throwing agent/error listener contained.

Rewrote the test that encoded the old (buggy) "turn/start listener throw
is rethrown, no turn/end" semantics to assert the balanced-turn contract,
and added regressions for the throwing-turn/end-listener flush and the
agent/error report. Updated Agent.inject JSDoc.

packages/agent-loop/src/loop.ts

@@ -170,7 +170,6 @@ async function runTurn(ctx: Context, agent: LoopAgent, handle: LoopHandle, turn:
 
   let reason: TurnEndReason = { kind: 'completed' }
   let step = 0
-  let turnStarted = false
   let turnEnded = false
   let stepOpen = false
   let errorReported = false
@@ -236,9 +235,10 @@ async function runTurn(ctx: Context, agent: LoopAgent, handle: LoopHandle, turn:
 
   try {
     // --- Turn boundary. Once turn/start is appended, a turn/end is owed no
-    // matter what throws below; the catch + closeTurn guarantee it.
+    // matter what throws below; the catch + closeTurn guarantee it (the catch
+    // decides "owed" from the log via isTurnOpen, so even a throwing turn/start
+    // listener — append pushes before notifying — still gets its turn/end).
     session.append('turn/start', { turn, trigger })
-    turnStarted = true
     // Record the queued user messages INSIDE the turn (after turn/start), so
     // every event in the log is turn-enclosed. turn/end is now owed, so a throw
     // while appending these is caught below and the turn is still closed.
@@ -320,12 +320,20 @@ async function runTurn(ctx: Context, agent: LoopAgent, handle: LoopHandle, turn:
     // Normal / inline-error loop exit: close the turn and notify.
     closeTurn(true)
   } catch (error: unknown) {
-    // A pre-turn throw (turn/start append) is owed no turn/end — rethrow to
-    // the backstop. Otherwise a boundary emit (turn-start, step-start, the
-    // normal-path turn-end) or other unhandled throw escaped: close any open
-    // step, choose the reason (disposal wins only if no error was reported),
-    // record the error, and close the turn WITHOUT re-emitting agent/turn-end.
-    if (!turnStarted) throw error
+    // Decide whether this turn was ever opened from the LOG, not a flag.
+    // Session.append pushes the event BEFORE notifying session/event listeners,
+    // so a throwing listener on the `turn/start` append leaves turn/start in the
+    // log even though execution never reached the lines after that append.
+    // Gating on a "turn started" boolean would skip turn/end and leave a
+    // permanently OPEN turn that poisons the next turn/replay (ADR 0017). We
+    // check the log for THIS turn's turn/start: present means a turn/end is owed
+    // (or was already appended — closeTurn/failTurn are idempotent, so running
+    // them again is a safe no-op that still preserves the disposed/error reason
+    // chosen below). Absent means the turn/start append threw BEFORE its push (a
+    // non-serializable trigger — impossible for our fixed trigger); nothing was
+    // opened, so rethrow to the runLoop backstop.
+    const turnStartLogged = session.events.some(e => e.type === 'turn/start' && e.data.turn === turn)
+    if (!turnStartLogged) throw error
     closeStep()
     // Choose the close reason. Disposal wins only if no error was already
     // reported: a turn disposed mid-step sets reason=disposed in the step-error