chore(scripts): treat an expression-built script name as forbidden too (#2146)

Copilot, first review:

- The guard read only literal names, so `npm run smoke:web:${{ matrix.browser }}`
  and `npm run local:${{ matrix.task }}` walked past it — the one form in which
  a forbidden engine reaches CI without anyone typing its name. It already
  refused an expression-valued SMOKE_BROWSER for that exact reason; this applies
  the reasoning consistently. Its limit is now stated: a fully opaque
  `npm run ${{ matrix.script }}` names no family and is not detectable.
- The module header claimed the gate adds "the non-Chromium engine passes";
  it adds Firefox only, and WebKit belongs to neither tier.
- eslint.config.js:33 still said `npm run ci` — the last stale reference.
- AGENTS.md read "everything from `smoke` rightward is `local:gate` only",
  which is two claims wrong: smoke and Storybook run in GitHub CI too, and
  `pack:verify` is in neither tier.

Signed-off-by: Cliff Hall <cliff@futurescale.com>
Signed-off-by: cliffhall <cliff@futurescale.com>

eslint.config.js

@@ -30,7 +30,7 @@ const typeAware = {
     // an awaited one minus four characters, and the unhandled rejection it
     // produces surfaces in a different test, in a different file, as a stack
     // pointing at SDK internals — which is how #1947 came to fail the whole
-    // `npm run ci` chain from two un-held `callTool` promises.
+    // `npm run local:gate` chain from two un-held `callTool` promises.
     "@typescript-eslint/no-floating-promises": "error",
   },
 };