docs(smoke): finish the CI-topology correction in README and the workflow

Round 1 fixed the three smoke headers and AGENTS.md but left the same
overstatement in the two files Copilot flagged as previously-missed:

- README said the matrix runs 'the remaining engines', which reads as
  Firefox AND WebKit and contradicts the very next paragraph. It now
  names the current matrix (Firefox alone) and points at that paragraph.
- The workflow's fail-fast comment cited a WebKit-vs-Firefox scenario the
  one-entry matrix cannot produce. It now justifies keeping the flag for
  when a second engine returns, matching the AGENTS.md wording.

Swept the whole tree for the remaining variants of the claim; none left.

Addresses the suppressed comments on Copilot's second review of #2133.

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

.github/workflows/main.yml

@@ -141,9 +141,10 @@ jobs:
   browser-engine-smokes:
     runs-on: ubuntu-latest
     strategy:
-      # Report every engine's verdict. Failing fast would hide a WebKit-only
-      # regression behind a Firefox-only one, which is the exact distinction
-      # this job exists to draw.
+      # Kept on despite the matrix currently holding one engine: the moment a
+      # second is added back (see below), failing fast would hide one engine's
+      # regression behind another's — which is the exact distinction this job
+      # exists to draw, so the flag should not have to be remembered then.
       fail-fast: false
       matrix:
         # `webkit` belongs here and is deliberately absent: the smokes RUN in it

README.md

@@ -479,7 +479,7 @@ SMOKE_BROWSER=webkit npm run smoke:web:app     # one smoke, one engine
 SMOKE_BROWSER=firefox npm run smoke:web:engine # all three smokes, one engine
 ```
 
-Unset, the engine is `chromium` — so `npm run ci` and `npm run smoke` are unchanged. CI runs Chromium inside the `build` job's `npm run smoke` and the remaining engines in a separate `Sandbox smokes (<engine>)` matrix job, rather than running the whole `build` job once per engine (validate, the coverage gate, the two verify gates and Storybook are all engine-independent). An unrecognized value is an error, not a fallback: a silent fallback would report a green Chromium run under a job labelled `webkit`. A missing browser binary fails naming the engine and its `npx playwright install --with-deps <engine>`.
+Unset, the engine is `chromium` — so `npm run ci` and `npm run smoke` are unchanged. CI runs Chromium inside the `build` job's `npm run smoke`, and every other gated engine in a separate `Sandbox smokes (<engine>)` matrix job — today that matrix holds **Firefox alone**; see the paragraph below for where WebKit stands. Splitting it that way, rather than running the whole `build` job once per engine, is deliberate: validate, the coverage gate, the two verify gates and Storybook are all engine-independent. An unrecognized value is an error, not a fallback: a silent fallback would report a green Chromium run under a job labelled `webkit`. A missing browser binary fails naming the engine and its `npx playwright install --with-deps <engine>`.
 
 **CI green today: Chromium and Firefox.** WebKit is supported by the tooling and worth running locally, but it is **not yet in the CI matrix** — the two App smokes fail there on a real, pre-existing Safari incompatibility in the web client's SSE transport, tracked with its diagnosis and a verified fix in [#2132](https://github.com/modelcontextprotocol/inspector/issues/2132). That is the matrix doing its job on its first outing: the bug affects the shipped web UI in Safari, and every Chromium-only tier had been green through it. An engine is either green in CI or absent from it — a `continue-on-error` job would report coverage nobody is held to.