fix(hosts): unsubscribe connection topics and keep the old socket until commit #88

Two control-plane defects in apps/control_plane/lib/workharbor/host_connections.ex:

1. The worker subscribes to PubSub topics (around line 103) and never unsubscribes. Each reconnect of a host adds a duplicate Registry entry, so :identity_changed arrives N times and triggers N sweeps. It grows over uptime.
2. The old connection is closed as :replaced (around lines 83-86) before the new slot transaction commits (around lines 92-101). If the transaction fails or the peer check turns false, the healthy socket is dropped and no new one is admitted.

Acceptance criteria

• Each host topic is subscribed at most once, or unsubscribed when the host has no remaining entry, with a test that reconnects repeatedly.
• The old connection closes only after the replacement commits, with a test for a failed replacement.
• make check is green.

Depends on #87 (same file).

Found by the independent pilot review on 2026-10-16. Builds on the pilot branch pilot/installable-pilot-pr (split from #83); start from its current head.

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