* fix(desktop): never idle-reap a local profile backend
Support f502bc6f: with a remote primary, a "This device" profile's local
`hermes serve` child was SIGTERM'd by the pool idle reaper after 60 min with
nothing streamed (the #105239 pinned-tier TTL), leaving "This device ·
Backend offline" while the user still had the chat open.
A local backend runs its profile's cron jobs and bot chats with nobody
watching, so idleness is not a reason to kill it. The idle reaper now only
reclaims connection descriptors (remote routes: no child, no slot). Local
children are still bounded by the maxBackends slot cap and foreground
reclaim (activeTurn vetoed), unchanged. Removes POOL_PINNED_IDLE_MS.
The Backend Idle Timeout setting description now says it applies to remote
connections only.
E2E no-idle-reap.spec.ts: a pooled local profile backend survives two reaper
ticks past both idle windows. Fails on main (backend reaped), passes here.
* chore(desktop): drop the dead pinned-tier idle policy (retireIdle, lastStreamedAt)
Greptile P2 on #134942: with local children no longer idle-reaped, retireIdle
has no production caller and lastStreamedAt no reader. Removes both, the stamp
write in touchPoolBackend, and the pinned-tier test. The shared retirement
safeguards stay covered through evictTo (identity, admission permit, activeTurn
veto).
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