feat(FN-4062): add global thinking log setting to persist agent reasoning a

Adds a global `thinkingLogEnabled` setting that gates AI thinking log persistence across the engine (executor, reviewer, merger, triage, step-session) and exposes the control in the dashboard Settings modal, with tests verifying settings parity and modal behavior.

Fusion-Task-Id: FN-4062

packages/engine/src/triage.ts

@@ -913,6 +913,7 @@ export class TriageProcessor {
           taskId: task.id,
           agent: "triage",
           persistAgentToolOutput: settings.persistAgentToolOutput,
+          persistAgentThinkingLog: settings.persistAgentThinkingLog,
           onAgentText: (id, delta) => {
             stuckDetector?.recordActivity(task.id);
             this.options.onAgentText?.(id, delta);