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/session-query/session-query-sqlite/tests/sqlite.spec.ts

@@ -340,7 +340,7 @@ describe('SQLite session search', () => {
       { type: 'user/message', seq: 2, time: 12, data: createUserMessage({
         content: [{ type: 'text', text: 'needle summary' }], source: { kind: 'plugin', plugin: 'test' },
       }), surfaceOp: { op: 'replace', start: 0, end: 0 }, sourceEventSeqs: [0] },
-      { type: 'turn/end', seq: 3, time: 13, data: { turn: 1, step: 1, reason: { kind: 'error', error: 'needle failure' } } },
+      { type: 'turn/end', seq: 3, time: 13, data: { turn: 1, step: 1, reason: { kind: 'error', error: { message: 'needle failure', code: 'UNKNOWN' } } } },
     ]
     ctx.sessions.create(SessionId('a'), { seed: events, meta: { cwd: '/a', parentSession: parent, createdAt: 20 } })
     ctx.sessions.create(SessionId('b'), { seed: messageEvents('needle peer', 12), meta: { createdAt: 20 } })