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/coverage-edges.spec.ts

@@ -155,7 +155,7 @@ describe('thrown-value propagation', () => {
     await waitForIdle(ctx, agent)
     const turnEnd = agent.session.events.find(e => e.type === 'turn/end')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error'
-      ? turnEnd.data.reason.error
+      ? turnEnd.data.reason.error.message
       : undefined).toBe('[object Object]')
   })
 })
@@ -464,7 +464,7 @@ describe('unrenderable failure settlement', () => {
     if (end?.type === 'turn/end' && end.data.reason.kind === 'error') {
       // The durable failure keeps the adapter facts' message, not the
       // unrenderable chain.
-      expect(errorChain(end.data.reason.error)).not.toBe('<unrenderable value>')
+      expect(errorChain(end.data.reason.error.message)).not.toBe('<unrenderable value>')
     }
   })
 })