feat(session): implement background job runner with queue and lifecycle events

[edlsh]

Implements the background job queue for async subagent delegation (#5887).

Stacked on #5996 — review/merge that first, then rebase this PR.

Changes:

• enqueue() with FIFO queue, deduplication, concurrency limit (default 2)
• Lifecycle events: session.background.{queued,started,completed,failed,canceled,timed_out}
• Timeout enforcement via Promise.race + AbortController
• cancel(), get(), list() helpers
• Memory pruning (max 100 completed jobs)
• Proper cleanup on Instance dispose
• Config: experimental.backgroundTasks.{timeoutMs,maxConcurrent}
• 25 integration tests