notify-slack: notification_message not rendered in Slack output

[swibrow]

Summary

The notification_message input is accepted but never rendered as visible text in the Slack message. Only the notification_title (header) and additional_fields are displayed.

Current behavior

The message step falls back to the title when building the text field:

MESSAGE="${{ inputs.notification_message }}"
if [ -z "$MESSAGE" ]; then
  MESSAGE="${{ inputs.notification_title }}"
fi

But $text is only used in the top-level text property (Slack's plain-text fallback for notifications/previews) — it's never added as a visible block. The Slack payload only contains:

1. A header block (title)
2. A section block with fields
3. A section block with workflow link

The notification_message is invisible in the actual Slack message.

Expected behavior

The notification_message should appear as a visible text block between the header and the fields section, e.g.:

{
  "type": "section",
  "text": {
    "type": "mrkdwn",
    "text": "go-service v1.15.1 has been released!"
  }
}

Additional issue

The additional_fields values don't support Slack mrkdwn link syntax. The Release field is passed as <url|text> but renders as plain text because the field values are interpolated via shell before reaching jq. The <> characters may be getting escaped or stripped.