feat(background-agent): add TaskHistory class for persistent task tracking

In-memory tracker that survives BackgroundManager's cleanup cycles.
Records agent delegations with defensive copies, MAX 100 cap per parent,
undefined-safe upsert, and newline-sanitized formatForCompaction output.

src/features/background-agent/task-history.ts

@@ -0,0 +1,75 @@
+import type { BackgroundTaskStatus } from "./types"
+
+const MAX_ENTRIES_PER_PARENT = 100
+
+export interface TaskHistoryEntry {
+  id: string
+  sessionID?: string
+  agent: string
+  description: string
+  status: BackgroundTaskStatus
+  category?: string
+  startedAt?: Date
+  completedAt?: Date
+}
+
+export class TaskHistory {
+  private entries: Map<string, TaskHistoryEntry[]> = new Map()
+
+  record(parentSessionID: string | undefined, entry: TaskHistoryEntry): void {
+    if (!parentSessionID) return
+
+    const list = this.entries.get(parentSessionID) ?? []
+    const existing = list.findIndex((e) => e.id === entry.id)
+
+    if (existing !== -1) {
+      const current = list[existing]
+      list[existing] = {
+        ...current,
+        ...(entry.sessionID !== undefined ? { sessionID: entry.sessionID } : {}),
+        ...(entry.agent !== undefined ? { agent: entry.agent } : {}),
+        ...(entry.description !== undefined ? { description: entry.description } : {}),
+        ...(entry.status !== undefined ? { status: entry.status } : {}),
+        ...(entry.category !== undefined ? { category: entry.category } : {}),
+        ...(entry.startedAt !== undefined ? { startedAt: entry.startedAt } : {}),
+        ...(entry.completedAt !== undefined ? { completedAt: entry.completedAt } : {}),
+      }
+    } else {
+      if (list.length >= MAX_ENTRIES_PER_PARENT) {
+        list.shift()
+      }
+      list.push({ ...entry })
+    }
+
+    this.entries.set(parentSessionID, list)
+  }
+
+  getByParentSession(parentSessionID: string): TaskHistoryEntry[] {
+    const list = this.entries.get(parentSessionID)
+    if (!list) return []
+    return list.map((e) => ({ ...e }))
+  }
+
+  clearSession(parentSessionID: string): void {
+    this.entries.delete(parentSessionID)
+  }
+
+  formatForCompaction(parentSessionID: string): string | null {
+    const list = this.getByParentSession(parentSessionID)
+    if (list.length === 0) return null
+
+    const lines = list.map((e) => {
+      const desc = e.description.replace(/[\n\r]+/g, " ").trim()
+      const parts = [
+        `- **${e.agent}**`,
+        e.category ? `[${e.category}]` : null,
+        `(${e.status})`,
+        `: ${desc}`,
+        e.sessionID ? ` | session: \`${e.sessionID}\`` : null,
+      ]
+      return parts.filter(Boolean).join("")
+    })
+
+    return lines.join("\n")
+  }
+}