ci: add typecheck and plain-Node ESM smoke test to PR checks

[bgub]

TL;DR

Adds two gates to CI that would have caught the release blockers fixed in #1827: a turbo typecheck step and a plain-Node ESM/CJS import smoke test for gt-next/config. Draft while #1827 is in review — this PR is stacked on that branch and should be retargeted to odysseus once it merges.

Code Changes

• .github/workflows/ci.yml: two new steps in the tests job, using the same change-based filters as the existing build/test steps:
  • Run typecheck (after build): turbo typecheck over changed packages and their dependents. The typecheck task already existed in turbo.json and every package has the script — it just never ran in CI. tsdown's dts emit passes unresolved re-exports through, so tsc --noEmit is the only thing that catches them.
  • Run smoke tests (after tests): runs the new smoke turbo task for changed packages that define one.
• turbo.json: new smoke task (dependsOn: ["build", "^build"], uncached). Build dependencies are cache hits by the time it runs in CI, so the added cost is negligible.
• packages/next/scripts/smoke.mjs + smoke script in package.json: imports gt-next/config via package self-reference from plain Node — await import() for the ESM build and createRequire for the CJS build — and asserts withGTConfig is exported. This exercises the exports map and module-evaluation-time code the way a real next.config.mjs does, which no bundled test app can (all in-repo apps use next.config.ts transpiled to CJS, which is why the require is not defined crash shipped unnoticed).

Notes / Flags

• Stacked on fix: repair broken type surfaces and ESM config entry before odysseus release #1827 (base is bg/fix-release-api-blockers): typecheck fails on plain odysseus until those fixes land (gt-react's initializeGT re-export, react-core's resolveStringContent options). Retarget to odysseus after merge.
• Verified locally with the exact CI commands against origin/odysseus: 35 typecheck tasks pass; the smoke task runs for gt-next and passes (and is a no-op for packages without a smoke script).
• The typecheck step covers the tests/apps/* workspace members (same filters as the test step, examples excluded). All pass today.
• Only gt-next has a smoke script for now; the same pattern would fit other packages whose entries are loaded by plain Node (e.g. the CLI), but that's left for follow-ups.