feat(compaction): add delegated agent sessions section with resume directive

Adds §8 to compaction prompt instructing the LLM to preserve spawned agent
session IDs and resume them post-compaction instead of starting fresh.
Injects actual TaskHistory data when BackgroundManager is available.

src/hooks/compaction-context-injector/index.test.ts

@@ -15,6 +15,7 @@ mock.module("../../shared/system-directive", () => ({
 }))
 
 import { createCompactionContextInjector } from "./index"
+import { TaskHistory } from "../../features/background-agent/task-history"
 
 describe("createCompactionContextInjector", () => {
   describe("Agent Verification State preservation", () => {
@@ -69,4 +70,47 @@ describe("createCompactionContextInjector", () => {
     expect(prompt).toContain("Do NOT invent")
     expect(prompt).toContain("Quote constraints verbatim")
   })
+
+  describe("Delegated Agent Sessions", () => {
+    it("includes delegated sessions section in compaction prompt", async () => {
+      //#given
+      const injector = createCompactionContextInjector()
+
+      //#when
+      const prompt = injector()
+
+      //#then
+      expect(prompt).toContain("Delegated Agent Sessions")
+      expect(prompt).toContain("RESUME, DON'T RESTART")
+      expect(prompt).toContain("session_id")
+    })
+
+    it("injects actual task history when backgroundManager and sessionID provided", async () => {
+      //#given
+      const mockManager = { taskHistory: new TaskHistory() } as any
+      mockManager.taskHistory.record("ses_parent", { id: "t1", sessionID: "ses_child", agent: "explore", description: "Find patterns", status: "completed", category: "quick" })
+      const injector = createCompactionContextInjector(mockManager)
+
+      //#when
+      const prompt = injector("ses_parent")
+
+      //#then
+      expect(prompt).toContain("Active/Recent Delegated Sessions")
+      expect(prompt).toContain("**explore**")
+      expect(prompt).toContain("[quick]")
+      expect(prompt).toContain("`ses_child`")
+    })
+
+    it("does not inject task history section when no entries exist", async () => {
+      //#given
+      const mockManager = { taskHistory: new TaskHistory() } as any
+      const injector = createCompactionContextInjector(mockManager)
+
+      //#when
+      const prompt = injector("ses_empty")
+
+      //#then
+      expect(prompt).not.toContain("Active/Recent Delegated Sessions")
+    })
+  })
 })