ci: drop the Firefox smoke job; keep the engines as an on-demand tool

The job was trialled and did not earn its place. It was genuinely cheap —
~2 minutes, running in parallel with the 15-minute build job, so zero
added wall-clock — but across a dozen runs Firefox never once disagreed
with Chromium. It was paying runner time and carrying a real flake
surface (playwright install --with-deps runs apt-get update, which fails
whenever a third-party repo in the runner image breaks; that is what the
one red run was) to re-confirm a result already in hand.

Everything that made cross-engine checking possible stays: SMOKE_BROWSER,
the shared headless-browser helper, the installer, and the tests. So
"SMOKE_BROWSER=firefox npm run smoke:web:engine" is there for the moment
it is worth running — before touching the sandbox, the CSP builder or the
proxy page — without taxing every push to re-derive the same answer.

The workflow is now byte-identical to v2/main again; the publish jobs go
back to needing only build.

Docs say what is true rather than what was intended, and AGENTS.md
records the reasoning so the job is not re-added on the general argument
that cross-engine coverage is good — that argument was already accepted,
and the on-demand path is what serves it. Re-add it on evidence: a real
cross-engine regression a gate would have caught.

Also swept the now-stale "matrix" language out of the smokes and the
resolver, including the empty-SMOKE_BROWSER rationale, which no longer
leans on a matrix key that does not exist.

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

scripts/pack-and-verify.mjs

@@ -567,7 +567,7 @@ async function verifyAppRender(baseUrl, token, whenWebServerExits) {
       label: LABEL,
     });
     // Chromium explicitly, not `resolveBrowserName()`: this is a *packaging*
-    // check, and the engine matrix (#2086) belongs to the smokes, where the
+    // check, and the engine question (#2086) belongs to the smokes, where the
     // sandbox surface is what is under test. Pinning it also means `pack:verify`
     // cannot be pointed at an engine its npm script never installed.
     browser = await loadBrowser(repoRoot, "chromium");