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/reviewer.ts

@@ -351,6 +351,7 @@ export async function reviewStep(
           ? (_id, delta) => options.onText!(delta)
           : undefined,
         persistAgentToolOutput: liveSettings?.persistAgentToolOutput,
+        persistAgentThinkingLog: liveSettings?.persistAgentThinkingLog,
       })
     : null;