ci(release): notify Slack when a prerelease succeeds #4604

Problem

Failed prerelease builds post a top-level Slack message to the triage channel, but successful ones post nothing. After a run of failures, there is no signal in the channel that a later attempt succeeded.

Proposal

Add a notify-prerelease-success job to .github/workflows/release-tag.yml that mirrors notify-prerelease-failure: it posts a top-level message to the same webhook when the prerelease succeeds, without the triage mention. Prereleases run about once a day, so the extra traffic is low; the job can be disabled if cadence increases.