chore(scripts): rename the local pre-push gate off ci, and guard it out of CI (#2146)

The root `ci` script was never what runs in GitHub CI, and the name worked
against us three ways: it collides with the built-in `npm ci` (which does
something else entirely and is one keystroke away), it invites the
cross-engine passes into a workflow by looking like the thing a workflow
should call, and it blurs "GitHub CI" and "the local gate" in every doc
that names it.

- Rename `ci` → `local:gate` and `ci:storybook` → `local:storybook`, with
  no back-compat alias: `npm run ci` now fails with npm's missing-script
  error, which prints the available scripts.

- Add `scripts/lib/workflow-gate.mjs` + its `test:scripts` suite, which
  runs over the repo's real `.github/workflows/**` and fails on an
  invocation of a `local:*` script, of a non-Chromium engine pass (derived
  from SUPPORTED_BROWSERS), or of `smoke:web:engine`, or on `SMOKE_BROWSER`
  set to anything but a literal `chromium`. It does not forbid
  `npm run smoke`, `smoke:web:chromium` or `smoke:tui` — those belong in CI
  and are there today. Comments are not scanned, so the workflow can still
  explain itself. This is the durable half; the rest is prose.

- Document the two-tier model once, in the root README ("Two tiers: GitHub
  CI and the local gate"), and point the other docs at it rather than
  restating the split.

- Sweep the ~40 stale references across README, AGENTS.md,
  .github/copilot-instructions.md, the client READMEs, the roadmap doc and
  the script headers. AGENTS.md and copilot-instructions.md change
  together, per the mirror rule.

No gate changes what it actually runs.

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

scripts/run-engine-smokes.test.mjs

@@ -73,7 +73,7 @@ describe("every engine tier consumes ENGINE_SMOKES", () => {
 
   it("the Firefox tier goes through the runner and is in the pre-push gate", () => {
     assert.match(scripts["smoke:web:firefox"], /run-engine-smokes\.mjs/);
-    assert.match(scripts.ci, /smoke:web:firefox/);
+    assert.match(scripts["local:gate"], /smoke:web:firefox/);
   });
 
   it("every supported engine has a `smoke:web:<engine>` script", () => {
@@ -92,7 +92,7 @@ describe("every engine tier consumes ENGINE_SMOKES", () => {
 
   it("each gated tier names its engine explicitly rather than reading the env", () => {
     // An ambient SMOKE_BROWSER must not be able to redirect a gate: without the
-    // literal engine, `SMOKE_BROWSER=firefox npm run ci` would run Firefox twice
+    // literal engine, `SMOKE_BROWSER=firefox npm run local:gate` would run Firefox twice
     // and never exercise Chromium at all.
     assert.match(
       scripts["smoke:web:chromium"],