feat(ci): add Slack failure notifications

[wyattjoh]

Summary

• Add scripts/slack.ts that posts block-kit formatted failure messages to a Slack incoming webhook
• Extract a reusable notify-failure.yml workflow to keep notification steps DRY across CI, stable release, and canary release pipelines

Requirements

Requires a SLACK_WEBHOOK_URL repository secret.

Test plan

• Verify notify-failure.yml is triggered when a CI job fails
• Verify Slack message format renders correctly
• Confirm no regressions in existing CI or release workflows

[wyattjoh]

Stack: slack-notifications

• feat(ci): add Slack failure notifications #151 👈 this PR
  • refactor(ci): extract reusable CI checks and setup-bun composite action #152

Part of a stacked PR chain. Do not merge manually.

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📝 Walkthrough

Walkthrough

Added a reusable GitHub Actions workflow .github/workflows/notify-failure.yml callable via workflow_call that accepts a required workflow-name input and a required SLACK_WEBHOOK_URL secret, checks out the repo, sets up Bun, installs dependencies, and runs bun scripts/slack.ts --status failure. Updated .github/workflows/release.yml to add notify-stable-failure and notify-canary-failure jobs that run the reusable workflow when specified upstream jobs include failures. Added scripts/slack.ts, a Bun script that requires --status, reads WORKFLOW_NAME and SLACK_WEBHOOK_URL from the environment, composes a Slack Block Kit payload with workflow and run metadata, and POSTs it to the webhook with error handling.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding Slack failure notifications to CI workflows.
Description check	✅ Passed	The description is clearly related to the changeset, detailing the specific changes made and test requirements.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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