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/smoke-web-app.mjs

@@ -31,7 +31,7 @@
  * file in the tarball packlist and on disk after a real install — and, since
  * #2003, drives phase 1's flow against the **installed** bin. Keep both:
  * pack:verify covers the published artifact, this covers the repo tree on every
- * `npm run ci`, where pack:verify (network-bound, local/release-only) does not
+ * `npm run local:gate`, where pack:verify (network-bound, local/release-only) does not
  * run — and this is the only one of the two that drives phase 2. Neither
  * subsumes the other.
  *
@@ -47,7 +47,7 @@
  * `webkit`). Three tiers, deliberately (#2086):
  *
  *   - **GitHub CI** runs this smoke in **Chromium** only.
- *   - **`npm run ci`**, the local pre-push gate, runs it in **Chromium and
+ *   - **`npm run local:gate`**, the local pre-push gate, runs it in **Chromium and
  *     Firefox** — the Firefox pass is `smoke:web:firefox`, and it is the one
  *     gate step with no GitHub CI counterpart.
  *   - **WebKit is on demand only**: `SMOKE_BROWSER=webkit npm run smoke:web:app`