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/core/src/settings-schema.ts

@@ -85,6 +85,7 @@ export const DEFAULT_GLOBAL_SETTINGS = {
   vitestKillThresholdPct: 90,
   // Agent log persistence controls
   persistAgentToolOutput: true,
+  persistAgentThinkingLog: false,
   researchGlobalDefaults: {
     searchProvider: undefined,
     synthesisProvider: undefined,