fix(background-agent): redact archived prompts and cap fallback archive

Store only sanitized completed-task snapshots in archive to avoid retaining sensitive prompts after cleanup.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/manager.test.ts

@@ -6014,7 +6014,41 @@ describe("BackgroundManager regression fixes - resume and aborted notification",
 
     //#then
     expect(getTaskMap(manager).has(task.id)).toBe(false)
-    expect(manager.getTask(task.id)?.sessionId).toBe(task.sessionId)
+    const archivedTask = manager.getTask(task.id)
+    expect(archivedTask?.sessionId).toBe(task.sessionId)
+    expect(archivedTask?.prompt).toBe("[redacted]")
+
+    manager.shutdown()
+  })
+
+  test("should cap completed task archive size at 100 entries", () => {
+    //#given
+    const manager = createBackgroundManager()
+
+    //#when
+    for (let index = 0; index < 120; index += 1) {
+      const task: BackgroundTask = {
+        id: `task-archive-${index}`,
+        sessionId: `session-archive-${index}`,
+        parentSessionId: "parent-session",
+        parentMessageId: "msg-1",
+        description: "archive cap regression",
+        prompt: `sensitive-${index}`,
+        agent: "explore",
+        status: "completed",
+        startedAt: new Date(),
+        completedAt: new Date(),
+      }
+      ;(cast<{ removeTask: (task: BackgroundTask) => void }>(manager)).removeTask(task)
+    }
+
+    //#then
+    const archive = cast<Map<string, unknown>>(Reflect.get(manager, "completedTaskArchive"))
+    expect(archive.size).toBe(100)
+    expect(archive.has("task-archive-0")).toBe(false)
+    expect(archive.has("task-archive-19")).toBe(false)
+    expect(archive.has("task-archive-20")).toBe(true)
+    expect(archive.has("task-archive-119")).toBe(true)
 
     manager.shutdown()
   })