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/acp/acp/src/index.ts

@@ -146,7 +146,7 @@ export function apply(ctx: Context, config: AcpConfig): void {
     inflight: NonNullable<SessionRecord['inflight']>,
     reason: Extract<TurnEndReason, { kind: 'error' }>,
   ): void => {
-    inflight.reject(internalError(`turn failed: ${errorChain(reason.error)}`))
+    inflight.reject(internalError(`turn failed: ${reason.error.message}`))
   }
 
   // Emit only committed assistant text. Raw chunks, reasoning, tools, plans,

packages/acp/acp/tests/codec.spec.ts

@@ -9,7 +9,7 @@ describe('ACP codec', () => {
     [{ kind: 'aborted', reason: { kind: 'user' } }, 'cancelled'],
     [{ kind: 'interrupted' }, 'cancelled'],
     [{ kind: 'blocked' }, 'end_turn'],
-    [{ kind: 'error', error: 'failed' }, 'end_turn'],
+    [{ kind: 'error', error: { message: 'failed', code: 'UNKNOWN' } }, 'end_turn'],
   ] satisfies Array<[TurnEndReason, string]>)('maps %o to %s', (reason, expected) => {
     expect(turnEndToStopReason(reason)).toBe(expected)
   })