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/prompt-builder.ts

@@ -31,6 +31,8 @@ interface PromptContext {
   activeSprint?: SprintRecord | null;
   recentDocuments?: DocumentRecord[];
   allConnectors?: Array<{ id: string; label: string; status: string }>;
+  /** D-07: Handoff markdown from a rotated session, injected as context for the agent */
+  sessionHandoff?: string | null;
 }
 
 export function buildSystemPrompt(ctx: PromptContext): string {
@@ -195,6 +197,15 @@ ${relevantProjects.map((p) => `- [${p.id}] ${p.name}: ${p.description || ""} (st
 `);
   }
 
+  // D-07: Inject handoff context from previous rotated session
+  if (ctx.sessionHandoff) {
+    sections.push(`# Session Handoff
+The following context was preserved from your previous session on this task:
+
+${ctx.sessionHandoff}
+`);
+  }
+
   if (ctx.task) {
     sections.push(`# Current Task
 - ID: ${ctx.task.id}