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/subagent/subagent-dsh-sdk/tests/subagent-dsh-sdk.spec.ts

@@ -74,7 +74,7 @@ describe('sdkStopReason', () => {
     expect(sdkStopReason({ kind: 'completed' })).toBe('completed')
     expect(sdkStopReason({ kind: 'max-tokens' })).toBe('max-tokens')
     expect(sdkStopReason({ kind: 'aborted', reason: { kind: 'user' } })).toBe('aborted')
-    expect(sdkStopReason({ kind: 'error', error: new Error('x') })).toBe('error')
+    expect(sdkStopReason({ kind: 'error', error: { message: 'x', code: 'UNKNOWN' } })).toBe('error')
     expect(sdkStopReason({ kind: 'interrupted' })).toBe('error')
     expect(sdkStopReason({ kind: 'aborted', reason: { kind: 'disposed' } })).toBe('aborted')
   })