fix(core): keep background shell locations alive - #52580

Supporting data for this PR — it fixes the notification-loss half of the eviction bug, not just the shell kill.

Independent repro on v2.0.22: background: true shell job launched at 18:02, worker idle at 18:03:05 (last durable event in the location), 60-min TTL evicts the location ~19:03, job actually finishes 19:11:33 — and no synthetic completion row is ever written (verified absent in session_message; session_inbox/session_pending empty). The .out file is also removed, matching the watcher's Effect.onInterrupt(() => shell.remove(...)) (tool/plugin/shell.ts:234). The waiting session stayed idle until woken manually 70 min later.

A second background job in the same session that ran < 60 min delivered normally, confirming the path works when the location survives.

Because notifyWhenDone is forkIn'd into the location/plugin scope, keeping the location's services cached while a backgrounded shell is live (as this PR does) is exactly what preserves sessions.synthetic(...) and the auto-resume. Kindly prioritise; this strands any background: true job that outlives the idle TTL, which is a common long-running-build/test pattern.

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