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/telemetry/session-telemetry/tests/telemetry.spec.ts

@@ -126,7 +126,7 @@ describe('TelemetryCoordinator capture', () => {
       }),
     }, { surfaceOp: 'append' })
     session.append('telemetry-test/opaque', { payload: { nested: [] } })
-    session.append('turn/end', { turn: 1, step: 1, reason: { kind: 'error', error: 'boom' } })
+    session.append('turn/end', { turn: 1, step: 1, reason: { kind: 'error', error: { message: 'boom', code: 'UNKNOWN' } } })
     const severities = backend.ledger().map(r => [r.attributes['event.type'], r.severity])
     expect(severities).toEqual([
       ['turn/start', 'info'],