feat(ci): add release Slack notifications

[kriscoleman]

feat(ci): add release Slack notifications

Summary

This PR adds a notify job to the release workflow that automatically sends a Slack notification via a reusable workflow whenever a new EKCO release is published.

Changes

• Added notify job to .github/workflows/release.yaml
• Job depends on the release job completing successfully
• Uses replicatedhq/reusable-workflows/.github/workflows/notify-release/notify-release.yml@main
• Passes the release tag and PRODUCTION_SYSTEMS_SLACK_WEBHOOK secret

UAT

1. Merge this PR
2. Create and push a new semver tag (e.g., v0.1.0)
3. Verify the release workflow runs successfully
4. Confirm a Slack notification is sent to the configured channel with the release tag

[xavpaice]

Is PRODUCTION_SYSTEMS_SLACK_WEBHOOK available for this repo already?

[kriscoleman]

Is PRODUCTION_SYSTEMS_SLACK_WEBHOOK available for this repo already?

Yes, I worked with Josh Sandlin on that, he added it at the ORG level, so it should be available globally within replicatedhq now