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/loop.spec.ts

@@ -235,7 +235,7 @@ describe('agent loop', () => {
     const turnEnd = agent.session.events.find(e => e.type === 'turn/end')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind).toBe('error')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error'
-      ? turnEnd.data.reason.error
+      ? turnEnd.data.reason.error.message
       : '').toContain('no value for this assembly')
 
     // The loop survived: a waterfall listener rescues {{cwd}} and the SAME