feat(ci): unified build-notification action for Flex Slack alerts

[y3rsh]

Overview

Formalizes Flex build Slack notifications into a single TypeScript action (build-notification), inspired by and intended to supersede:

• fix(slack,cache): changed order of cache sync #333 (consolidated alerts, deployed status, clearer payload fields)
• chore: update slack github action version to remove deprecated cmd warning #284 (safe payload construction, modern Slack/action patterns)

Before: four notify-* jobs in build-ot3-actions.yml and a composite simple-build-alert action with workflow-level conditionals and duplicated inputs.

After: one notify-build job (if: always()) that passes all context; the action decides whether to send, message shape (deployed / failure / cancelled), and webhook destination (webhook_url vs webhook_url_tagged).

Follows the same pattern as build-refs: Node 24 action, unit tests, lint/format CI, and a check that committed dist/index.js is up to date.

Changelog

• Added .github/actions/build-notification with tested routing rules and bundled dist/index.js
• Added .github/workflows/test-build-notification.yml CI for the action
• Replaced four notify jobs in build-ot3-actions.yml with one notify-build job
• Removed .github/actions/simple-build-alert
• Upgraded @actions/core to v3.x (fixes transitive undici audit findings)

Review requests

• Confirm Slack workflow payloads still match expected fields (tag, headline, status, artifact URLs)
• Confirm webhook routing (SLACK_WEBHOOK_URL vs SLACK_WEBHOOK_URL_TAGGED) matches current channel expectations
• OK to close fix(slack,cache): changed order of cache sync #333 and chore: update slack github action version to remove deprecated cmd warning #284 once this merges?

Risk assessment

Attention level: medium

Trade-offs: notification timing moves to end of workflow (single job) rather than inline after S3 upload; cache sync order unchanged from main.

Blast radius: Flex build Slack notifications only; no BitBake/build logic changes.

Mitigation: 13 unit tests for routing rules; CI verifies dist bundle; continue-on-error: true on notify step preserves build job outcome.

[neo-jesse]

move-cache-sync fixes artifact upload order inside the job; it does not move Slack earlier. For “Slack right after deploy, before cache,” you’d still need an in-run-build step after upload (or merge move-cache-sync and add that step). -- is this the expected behavior now?