feat(KB-119): add configurable thinking effort level

- Add defaultThinkingLevel to core Settings types and exports
- Thread thinking level through engine executor, merger, reviewer, triage, and pi
- Add effort dropdown to Model settings UI in SettingsModal
- Add tests for thinking effort dropdown in SettingsModal

packages/engine/src/merger.ts

@@ -239,6 +239,7 @@ export async function aiMergeTask(
     onToolStart: agentLogger.onToolStart,
     defaultProvider: settings.defaultProvider,
     defaultModelId: settings.defaultModelId,
+    defaultThinkingLevel: settings.defaultThinkingLevel,
   });
 
   try {