chore(scripts): parse the workflow instead of matching lines (#2146)

Copilot, third review — and the second finding is really a verdict on the
approach, so the fix is the approach:

- `SMOKE_BROWSER:` with no value is valid YAML that sets the variable to the
  empty string, which `resolveBrowserName` rejects as present-but-empty rather
  than treating as unset. The value pattern required \S+, so the guard said
  nothing. It is \S* now, with its own message.

- The hand-rolled opener could not see `run: | # explanation` or the flow
  mapping `env: { SMOKE_BROWSER: firefox }`. That is the third spelling it has
  missed in three rounds, each one a forbidden invocation that would have
  passed while test:scripts stayed green — worse than no guard, because it
  reports a coverage that is not there. YAML has too many spellings to
  recognize by indentation, so extractExecutableRegions now parses with the
  `yaml` package. It is already a root dependency, and this file is not in the
  published files allowlist, so nothing about the tarball changes.

A parse error now throws rather than yielding no regions: a workflow the guard
cannot read must not pass as a workflow with nothing in it.

Verified by injecting a flow-mapping SMOKE_BROWSER override into the real
main.yml — the suite fails, and goes green again when removed.

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

Author: cliffhall

AGENTS.md

@@ -1003,7 +1003,7 @@ Everything about the selection lives in **`scripts/lib/headless-browser.mjs`** 
 - **An unrecognized `SMOKE_BROWSER` is an error, never a fallback.** Falling back to Chromium would report a green Chromium run under a job labelled `webkit` — coverage claimed but not run, which is worse than none.
 - **A launch failure names the engine that failed** and a remedy that works **from where the caller actually is**: `npm run smoke:web:<engine>`, or `npx playwright install --with-deps <engine>` run *from `clients/web`*. Naming `chromium` while WebKit was missing would send the reader to install a browser they already have — and naming a bare root-level `npx playwright` would send them to a version that is not the pinned one, which can install a browser revision the pinned Playwright still cannot launch.
 - **Firefox is gated locally, not in CI, and that asymmetry is deliberate.** A GitHub Actions job was trialled and removed. It was cheap — ~2 minutes, parallel with the 15-minute `build` job, so zero added wall-clock — but across a dozen runs Firefox never once disagreed with Chromium, so it spent runner minutes on every push, from every branch, 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) to re-confirm a result already in hand. Moving it into `npm run local:gate` keeps the check where a human is about to push a change they can still reason about, and pays for it once rather than on every push. **Don't re-add the CI job on the argument that cross-engine coverage is good in principle** — that argument was accepted, and the pre-push gate is what serves it. Re-add it on evidence: a cross-engine regression that reached `v2/main` because someone skipped the gate.
-- **A guard enforces the split, so it cannot be undone by accident (#2146).** `scripts/lib/workflow-gate.mjs` is run by `npm run test:scripts` over every file under `.github/workflows/`, and fails the suite on: an invocation of a `local:*` script (the prefix *means* local-only, so a future sibling is covered without editing the guard); an invocation of a non-Chromium engine pass, derived from `SUPPORTED_BROWSERS` so a fourth engine extends the guard rather than opening a hole in it; an invocation of `smoke:web:engine`, whose engine comes from the environment and so cannot be read off the workflow at all; or `SMOKE_BROWSER` set to anything but a literal `chromium` — the back door that would redirect an otherwise innocent `npm run smoke`. A name built from a workflow **expression** counts as forbidden too (`smoke:web:${{ matrix.browser }}`, `local:${{ matrix.task }}`): a matrix the guard cannot read may well contain `firefox`, so an unreadable name is not waved through. Its limit is worth knowing — a fully opaque `npm run ${{ matrix.script }}` names no family at all and is not detectable. It deliberately does **not** forbid `npm run smoke`, `smoke:web:chromium`, or `smoke:tui`: those belong in CI and are there today, and `smoke:tui` self-skips under `process.env.CI` on its own. It reads only the positions that execute something — `run:` scalars (inline and block) and the values of an `env:` or `with:` mapping — so ordinary metadata such as a step `name:` explaining the rule is not reported as an invocation of it (Copilot); comments are skipped for the same reason, and because a guard that made the workflow undocumentable would be traded for the wrong thing. That scoping is a stated limit rather than a hidden one: an execution vector outside those three positions is not seen.
+- **A guard enforces the split, so it cannot be undone by accident (#2146).** `scripts/lib/workflow-gate.mjs` is run by `npm run test:scripts` over every file under `.github/workflows/`, and fails the suite on: an invocation of a `local:*` script (the prefix *means* local-only, so a future sibling is covered without editing the guard); an invocation of a non-Chromium engine pass, derived from `SUPPORTED_BROWSERS` so a fourth engine extends the guard rather than opening a hole in it; an invocation of `smoke:web:engine`, whose engine comes from the environment and so cannot be read off the workflow at all; or `SMOKE_BROWSER` set to anything but a literal `chromium` — the back door that would redirect an otherwise innocent `npm run smoke`, and one that includes setting it to *nothing*, since present-and-empty is rejected rather than defaulted. A name built from a workflow **expression** counts as forbidden too (`smoke:web:${{ matrix.browser }}`, `local:${{ matrix.task }}`): a matrix the guard cannot read may well contain `firefox`, so an unreadable name is not waved through. Its limit is worth knowing — a fully opaque `npm run ${{ matrix.script }}` names no family at all and is not detectable. It deliberately does **not** forbid `npm run smoke`, `smoke:web:chromium`, or `smoke:tui`: those belong in CI and are there today, and `smoke:tui` self-skips under `process.env.CI` on its own. It **parses** the workflow (via the root-declared `yaml` package) and reads only the positions that execute something — `run:` scalars (inline and block) and the values of an `env:` or `with:` mapping — so ordinary metadata such as a step `name:` explaining the rule is not reported as an invocation of it (Copilot); comments are skipped for the same reason, and because a guard that made the workflow undocumentable would be traded for the wrong thing. That scoping is a stated limit rather than a hidden one: an execution vector outside those three positions is not seen.
 - **This makes `npm run local:gate` a strict superset of GitHub CI rather than a mirror of it.** That was already the direction (`smoke:tui` self-skips on CI and runs only locally); Firefox is the second such step. The invariant that still holds, and the one that matters, is the useful direction: **passing `npm run local:gate` locally means CI's gates will pass.**
 - **`pack:verify` stays pinned to Chromium**, passing it explicitly rather than reading `SMOKE_BROWSER`. It is a _packaging_ check; the engine question belongs where the sandbox is under test, and pinning it also means it can't be pointed at an engine its npm script never installed.
 - **WebKit fails the two App smokes**, for reasons nobody has identified — the failure stage even varies between runs. The investigation was dropped as not worth the effort once it became clear it does not reproduce in real Safari, and that an isolated repro of the suspected mechanism did not reproduce it under Playwright's WebKit either. Treat a WebKit failure as unexplained rather than as a defect until someone has actually looked.

README.md

@@ -502,7 +502,7 @@ So the direction that matters holds: **passing `npm run local:gate` means CI's g
 
 **The gate is deliberately not named `ci`** ([#2146](https://github.com/modelcontextprotocol/inspector/issues/2146)). That name collided with `npm ci` — a built-in that clean-installs from the lockfile, does **not** run this script, and is one keystroke away — and it invited the cross-engine passes into GitHub CI by looking like the thing a workflow ought to call. There is **no back-compat alias**: `npm run ci` now fails with npm's missing-script error, which prints the available scripts.
 
-That is the readable half, and prose rots. The enforced half is `scripts/lib/workflow-gate.mjs`, run by `npm run test:scripts` over every file under `.github/workflows/`. It fails the suite if a workflow invokes a `local:*` script, a non-Chromium engine pass (`smoke:web:firefox`, `smoke:web:webkit`), or `smoke:web:engine` (whose engine comes from the environment and so cannot be read off the workflow at all), or sets `SMOKE_BROWSER` to anything but a literal `chromium`. It treats a name built from a workflow **expression** the same way — `smoke:web:${{ matrix.browser }}`, `local:${{ matrix.task }}` — since a matrix the guard cannot read may well contain `firefox`. It deliberately does **not** forbid `npm run smoke`, `smoke:web:chromium`, or `smoke:tui` — those belong in CI and are there today. It reads only the positions that actually execute something — `run:` scalars and the values of an `env:` or `with:` mapping — so a step *name* explaining the rule is not mistaken for an invocation of it, and the workflow can still document itself.
+That is the readable half, and prose rots. The enforced half is `scripts/lib/workflow-gate.mjs`, run by `npm run test:scripts` over every file under `.github/workflows/`. It fails the suite if a workflow invokes a `local:*` script, a non-Chromium engine pass (`smoke:web:firefox`, `smoke:web:webkit`), or `smoke:web:engine` (whose engine comes from the environment and so cannot be read off the workflow at all), or sets `SMOKE_BROWSER` to anything but a literal `chromium` (including setting it to *nothing*, which is present-and-empty rather than unset, and is rejected rather than defaulted). It treats a name built from a workflow **expression** the same way — `smoke:web:${{ matrix.browser }}`, `local:${{ matrix.task }}` — since a matrix the guard cannot read may well contain `firefox`. It deliberately does **not** forbid `npm run smoke`, `smoke:web:chromium`, or `smoke:tui` — those belong in CI and are there today. It **parses** the workflow rather than matching lines, and reads only the positions that actually execute something — `run:` scalars and the values of an `env:` or `with:` mapping — so a step *name* explaining the rule is not mistaken for an invocation of it, and the workflow can still document itself.
 
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