feat(investigation): wire up fastInvestigations from settings to backend

This completes the wiring so the Fast Mode toggle in the UI actually
takes effect for investigations.

Changes:
- Updated buildRunnerArgs to accept fastMode option and add --fast-mode flag
- Updated investigation handler to read fastInvestigations setting and pass it through

The complete flow is now:
1. User toggles Fast Mode in UI → saves to InvestigationSettings.fastInvestigations
2. Investigation starts → handler reads setting → passes to buildRunnerArgs
3. buildRunnerArgs adds --fast-mode CLI flag when enabled
4. CLI parses flag → GitHubRunnerConfig.fast_mode
5. Config passed to orchestrators → SDK clients created with fast_mode=true

This enables Opus 4.6 Fast Mode (2.5x faster, higher cost) for investigations.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

apps/frontend/src/main/ipc-handlers/github/investigation-handlers.ts

@@ -1173,12 +1173,16 @@ async function runInvestigation(
         // Non-fatal: will start fresh
       }
 
+      // Read investigation settings to get fastInvestigations flag
+      const investigationSettings = getInvestigationSettings(projectId);
+
       const args = [
         ...buildRunnerArgs(
           getRunnerPath(backendPath),
           project.path,
           'investigate',
           [String(issueNumber)],
+          { fastMode: investigationSettings.fastInvestigations ?? false },
         ),
         '--specialist-config', JSON.stringify(specialistConfig),
         ...resumeSessionsArg,

apps/frontend/src/main/ipc-handlers/github/utils/subprocess-runner.ts

@@ -765,6 +765,7 @@ export function buildRunnerArgs(
   options?: {
     model?: string;
     thinkingLevel?: string;
+    fastMode?: boolean;
   }
 ): string[] {
   const args = [runnerPath, '--project', projectPath];
@@ -777,6 +778,10 @@ export function buildRunnerArgs(
     args.push('--thinking-level', options.thinkingLevel);
   }
 
+  if (options?.fastMode) {
+    args.push('--fast-mode');
+  }
+
   args.push(command);
   args.push(...additionalArgs);