fix(ci): replace private actions with inline equivalents, add failure notifications

[benminer]

Summary

• Root cause: scope3data/actions is a private repo, but agentic-client is public — GitHub blocks public repos from using actions defined in private repos, causing "Unable to resolve action" failures in drift-detection and regenerate-schemas workflows
• Replaced scope3data/actions/node/install with actions/setup-node@v6 + npm ci in both workflows
• Replaced scope3data/actions/slack/post with a new local composite action at .github/actions/slack-post/action.yml that wraps the Slack chat.postMessage API
• Added failure notifications (if: failure()) to drift-detection, regenerate-schemas, and release workflows — all post to #agentic-service-alerts

Composite action details

The slack-post action takes channel, header, and content (Block Kit JSON array) as inputs, with SLACK_TOKEN passed via env. It automatically prepends a header block and repo context, appends a timestamp footer, validates the content is a JSON array, and checks the Slack API response for errors. All inputs are passed through env vars to avoid shell injection.

Test plan

• Trigger drift-detection via workflow_dispatch — should resolve actions and run successfully
• If drift is detected, verify Slack notification posts to #agentic-service-alerts with correct formatting
• Trigger regenerate-schemas via workflow_dispatch to verify node setup and Slack notification
• Verify failure notifications work (temporarily inject exit 1 before the failure handler, or check next natural failure)