fix(session): type turn/end error as one structured failure

TurnEndReasonMap.error now carries a single `error: LlmFailure` field:
an LlmError keeps its structured facts, any other error flattens to
errorChain text under the UNKNOWN code. Consumers read message/code
directly instead of defending against an unknown union — this also fixes
errorChain() rendering structured failures as '[object Object]' in the
TUI and ACP error paths. Document the turn-stopping contract: a
concludesTurn result never short-circuits already-submitted next-step
work (same-step additionalContexts or racing steering still runs), data
decides.

packages/core/agent-loop/tests/tool-calls.spec.ts

@@ -683,7 +683,7 @@ describe('tool-call scheduler: failure quiescence', () => {
     expect(turnEndBeforeDrain).toBeUndefined()
     expect(gated.pending()).toEqual([])
     expect(events(agent).findLast(event => event.type === 'turn/end')).toMatchObject({
-      data: { step: 1, reason: { kind: 'error', error: schedulerError.message } },
+      data: { step: 1, reason: { kind: 'error', error: { message: schedulerError.message, code: 'UNKNOWN' } } },
     })
   })
 })