Note

Low Risk
CI-only change to how the Slack webhook is passed; no application or security logic is affected.

Overview
Updates the announce-release job in publish-release.yml so slackapi/slack-github-action v3 gets the incoming webhook via with instead of environment variables.

The step now sets webhook-type: incoming-webhook and webhook: ${{ secrets.SLACK_WEBHOOK_URL }}, and drops the old SLACK_WEBHOOK_URL / SLACK_WEBHOOK_TYPE env block. Release Slack message content and continue-on-error behavior are unchanged.

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