[RFE] potential improvements to GHA

[heitbaum]

List of potential improvements to GHA

Pending:

• tools-clean-img.yml trigger: workflow_run: [build-images] is dead. Needs investigation: does NIGHTLY_UPLOAD_PATH = /var/www/test? Are tools-clean-img (keep 2) and tools-cleanup-images (keep 200 via prune-archive.py) complementary or contradictory? Resolve before fixing trigger.
• SPDX/licensing in workflow YAML files: scripts done; .github/workflows/*.yml and .github/actions/ files not yet covered

Future:

• make-image.yml: capture build errors into ENV, add step before "Upload OFFICIAL Release image" to surface failures
• create-addon.yml: notify about addons that did not build successfully
• Slack integration: build notices + addon build failures
• Slack: migrate to slackapi/slack-github-action@v3
• Build host /source/ cleanup: prune source tarballs (.tgz, .sha256, .url) on self-hosted build servers, keeping only files referenced by stable and master branches — disk space management
• deploy-addons.yml: replace raw version number input with a human-readable LE release dropdown (e.g. LE 12, LE 12.2, LE 13) that maps to the correct addon repo version number behind the scenes
• README / docs overhaul: update active branch description (12.0 EOL → 12.2 + 13), fix badge labels to match renamed workflows, complete incomplete sections (bug reports, contributing); review docs/ for accuracy

Known issues:

• actionlint false positive on client-id in actions/create-github-app-token@v3
  • feat: add client-id input and deprecate app-id actions/create-github-app-token#353
  • False-positive on actions/create-github-app-token@v3 client-id input rhysd/actionlint#669
  • actions/create-github-app-token@v3 inputs changed rhysd/actionlint#648
  • Update popular_actions.go for actions/create-github-app-token@v3 rhysd/actionlint#668