feat: notify Slack on build failure

[stackptr]

Adds a Slack notification to the build job that fires on failure, for both PR and push-to-main contexts.

Changes

• New Prepare Slack context step computes the trigger string (PR or push) and short SHA as step outputs
• New Notify Slack on failure step uses slackapi/slack-github-action@v2 with a Block Kit payload: red-bordered attachment, header, two-column section (host + trigger), and a context line with linked repo, commit, and run URL
• Requires a SLACK_WEBHOOK_UPDATES_CI repository secret (incoming webhook for #updates)

Verify

• Confirm SLACK_WEBHOOK_UPDATES_CI secret is set before merging
• Trigger a deliberate build failure on a PR and a push to main to confirm both message shapes render correctly in Slack

[stackptr]

• feat: notify Slack on build failure #454 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.