Summary

Connect the nightly E2E scorecard workflow (#2613) to a Slack channel so the team gets a morning CI health summary without opening GitHub.

Parent tracking issue: #2612
Depends on: #2613 (scorecard workflow)

Setup Steps

Design

The scorecard workflow (#2613) already includes a conditional Slack posting hook:

• If SLACK_WEBHOOK_URL secret exists → format and POST scorecard as Slack Block Kit message
• If secret is missing → skip silently, scorecard still appears in GitHub Actions job summary

This issue covers:

1. The Slack admin setup (channel + webhook + secret)
2. Formatting the scorecard as a Slack Block Kit message with proper sections, emoji, and links
3. End-to-end verification

Slack Message Format

Use Block Kit for rich formatting:

• Header block with date
• Section block with run summary (perfect / failed / cancelled)
• Section block with top flaky jobs
• Section block with trend arrow
• Actions block with link to GitHub Actions

Future Enhancements (out of scope)

• Tag on-call in Slack when failure rate exceeds a threshold
• 7-day rolling trend chart
• Weekly rollup summary
• Thread replies with per-run details

Acceptance Criteria

• Slack channel exists and team has access
• SLACK_WEBHOOK_URL repo secret is configured
• Scorecard posts to Slack daily at ~8am ET
• Slack message includes: run counts, perfect count, flaky jobs, trend, link to Actions
• Manual workflow_dispatch triggers Slack post for testing
• No Slack errors when webhook is valid