fix(core): keep background shell locations alive

[opencode-agent]

Summary

• Keep a Location's services cached while a shell job started there remains backgrounded and running. Capture the shell's owning Location at job start rather than inferring it later from a Session that may have moved.
• Recheck for shells admitted during eviction cleanup before detaching the cached graph.
• Do not pin a parent Location just because it is waiting on a background sub-agent in another Location; completion enters the parent's durable inbox and wakes it normally.

Validation

• SHELL=/bin/bash bun test test/location-activity.test.ts test/browser-idle.test.ts test/job.test.ts test/tool-shell.test.ts (88 pass, 42 platform skips)
• bun typecheck from packages/core
• bun --bun run lint from repo root
• Full bun run check unavailable on this host's Node 18 (Astro requires Node >=22.12); attempting the whole-workspace check with Bun also hit a host-wide concurrent typecheck crash. Scoped checks above passed.

Requested by: @jlongster (James via Slack)

[SamSpiri]

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.