fix(background): scope task notifications to originating parent session

[filipeandre]

Summary

• fix background task completion aggregation so notifications are scoped to the originating parent session instead of all historical tasks
• route completion reminders to the task owner session and detach deleted parent sessions safely to avoid cross-session notification bleed
• add a regression test suite for BackgroundManager and a test npm script for easier verification

Why

Background reminders could include task IDs from other sessions because task state was globally aggregated and notification routing used mutable main-session state.

Validation

• bun test tests/background-manager.test.ts
• bun run typecheck
• bun run build