feat(compaction): wire TaskHistory into BackgroundManager and compaction pipeline

Records task history at 6 status transitions (pending, running×2, error,
cancelled, completed). Exports TaskHistory from background-agent barrel.
Passes backgroundManager and sessionID through compaction hook chain.

src/features/background-agent/index.ts

@@ -1,4 +1,5 @@
 export * from "./types"
 export { BackgroundManager, type SubagentSessionCreatedEvent, type OnSubagentSessionCreated } from "./manager"
+export { TaskHistory, type TaskHistoryEntry } from "./task-history"
 export { ConcurrencyManager } from "./concurrency"
 export { TaskStateManager } from "./state"