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/session/tests/fork.spec.ts

@@ -139,7 +139,7 @@ describe('SessionStore.fork', () => {
     const reasons: TurnEndReason[] = [
       { kind: 'completed' },
       { kind: 'aborted', reason: { kind: 'user' } },
-      { kind: 'error', error: 'model failed' },
+      { kind: 'error', error: { message: 'model failed', code: 'UNKNOWN' } },
       { kind: 'aborted', reason: { kind: 'disposed' } },
       { kind: 'max-tokens' },
       { kind: 'interrupted' },

packages/core/session/tests/invariant.spec.ts

@@ -331,7 +331,7 @@ describe('session-log invariants', () => {
       unresolved.append('step/start', { turn: 1, step: 1 })
       unresolved.append('tool/call', { turn: 1, step: 1, callId: CallId('c1'), name: 'echo', arguments: '{}' })
       unresolved.append('step/end', { turn: 1, step: 1 })
-      unresolved.append('turn/end', { turn: 1, step: 1, reason: { kind: 'error', error: 'boom' } })
+      unresolved.append('turn/end', { turn: 1, step: 1, reason: { kind: 'error', error: { message: 'boom', code: 'UNKNOWN' } } })
     }).not.toThrow()
   })