docs(smoke): state the actual CI browser coverage, not the intended one

The three smoke headers and one AGENTS.md bullet still said CI runs all
three engines. That was written before pointing the smokes at WebKit
found #2132; the matrix was then narrowed to Firefox and these were not
brought along, so they claimed a gate that does not exist. Each now says
what CI actually enforces — Chromium in the build job's `npm run smoke`,
Firefox in the Sandbox smokes matrix — and where WebKit stands.

smoke-web-browser.mjs additionally notes that IT passes in WebKit; the
two App smokes are the ones that do not, and a reader of this file should
not infer otherwise from a bare '#2132' reference.

The fail-fast: false rationale no longer cites a WebKit-vs-Firefox
example it cannot currently produce, and says why the flag is still right
for a one-entry matrix.

Addresses Copilot review on #2133.

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

scripts/smoke-web-browser.mjs

@@ -46,8 +46,12 @@
  * Launching the browser (and resolving Playwright from clients/web, which has
  * its own gotcha — see `lib/headless-browser.mjs`) is delegated to that module,
  * which is also where `SMOKE_BROWSER` picks the engine: `chromium` (the
- * default), `firefox`, or `webkit`, all three of which CI runs (#2086). The
- * engine question here is narrower than in the App smokes — this asserts a clean
+ * default), `firefox`, or `webkit` (#2086). CI gates Chromium (in the `build`
+ * job's `npm run smoke`) and Firefox (in the `Sandbox smokes` matrix job);
+ * WebKit runs but is not gated yet, pending #2132 — note this smoke PASSES in
+ * WebKit, it is the two App smokes that do not.
+ *
+ * The engine question here is narrower than in the App smokes — this asserts a clean
  * first paint, i.e. that the shipped bundle's syntax and API level are
  * *reachable* on the engine at all, rather than anything about the sandbox — but
  * it is real, and it is nearly free to include.