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/support/llm-replay/tests/llm-replay.spec.ts

@@ -183,7 +183,7 @@ describe('deriveReplayScript', () => {
     const events: SessionEvent[] = [
       chunkEvent(1, 1, 1, { type: 'block-start', index: 0, blockType: 'text' }),
       chunkEvent(2, 1, 1, { type: 'text-delta', index: 0, text: 'par' }),
-      { type: 'turn/end', seq: 3, time: 0, data: { turn: 1, step: 1, reason: { kind: 'error', error: 'x' } } },
+      { type: 'turn/end', seq: 3, time: 0, data: { turn: 1, step: 1, reason: { kind: 'error', error: { message: 'x', code: 'UNKNOWN' } } } },
     ]
     expect(() => deriveReplayScript(events)).toThrow(/without a finish chunk.*replay\.override\.json/s)
   })