feat(06-02): wire session handoff injection into prompt builder and orchestration dispatch

- Add sessionHandoff field to PromptContext interface and Session Handoff section in buildSystemPrompt
- Add getSessionHandoff dependency to orchestration service, load handoff before prompt build
- Update orchestration to prefer task_sessions table over legacy tasks.session_json (SESS-01)
- Wire getSessionHandoff through ServiceGraph into createOrchestrationService

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/service-graph.ts

@@ -234,6 +234,13 @@ export function createServiceGraph({
         };
       })(),
       budgetService: getOrCreateBudgetEnforcementService(),
+      getSessionHandoff: (taskId: string) => {
+        try {
+          return getOrCreateSessionManagementService().getHandoffContext(taskId);
+        } catch {
+          return null;
+        }
+      },
     });
     workerService.createWorker();