Investigate why postbuild:apps icon guard didn't catch missing header icons #706

Context

scripts/check-icon-imports.ts runs as part of postbuild:apps and DOES catch missing icons via icon="…" attributes. Running it locally against current main returns FAIL with four unregistered icons on header.html (learning-assistant, action, chain-link, discussion).

That means either:

• A. The deploy that shipped the broken icons bypassed postbuild:apps (CI flag, manual override, local mbt build without npm run build:all)
• B. The icons were added AFTER the last good guard run
• C. postbuild:apps was wired into the wrong build chain (e.g. build but not build:all)

Action

1. Walk the deploy chain in .github/workflows/deploy.yml, mta.yaml build-results, and the npm script tree to find where postbuild:apps does/doesn't fire
2. Check git blame on the four affected header.html lines vs the last deploy that ran postbuild:apps
3. Add the guard run as an explicit step in deploy.yml if it's currently only an npm postbuild hook (which can be silently skipped with --ignore-scripts or a separate mbt build invocation)

Companion PR fixes the icons + broadens the guard regex to also catch <ui5-icon name="…"> usages: TBD