feat: notify main agent when background agents finish

[RadicalGitter]

Summary

• emit a one-shot notification when a background agent completes, fails, or is killed
• scope completion delivery to the owning session and surface it at the next main-agent turn
• point the model to the task output file instead of injecting the full child result into context
• document the behavior and cover completion, deduplication, session isolation, and terminal transitions

Motivation

Background shell jobs already surface <task-notification> messages, while background agent jobs currently require the parent to poll TaskOutput. This adds the same compact completion signal for agents without treating child output as accepted or consuming another session's event.

Verification

• bun run format:check
• bun run lint (0 errors; repository baseline warnings only)
• bun run typecheck
• bun run test (427/427 workspace test files passed)
• bun run build