ci: post release summaries to Slack

[PabloPardoGarcia]

Adds a notify job to the release workflow that posts the changelog to Slack whenever a version ships.

Design notes:

• Runs needs: [release-please, publish], i.e. only after the PyPI publish succeeds — the channel never sees a release that failed to ship.
• Placed inside the existing workflow rather than a separate on: release workflow, which would never fire: release-please creates the GitHub Release with GITHUB_TOKEN, and events from that token don't trigger other workflows (GitHub's recursion guard).
• Fetches the release body via gh release view and converts release-please's markdown to Slack mrkdwn (headings/bold/links), truncated at 3500 chars.
• No-ops politely until the SLACK_WEBHOOK_RELEASES repo secret exists, so merging is safe before the secret is set.
• Payload conversion tested locally against a real release-please changelog shape.

Setup required after merge: add the incoming-webhook URL as repo secret SLACK_WEBHOOK_RELEASES.