ci: run typecheck + Vitest on every push to main/feat/** and on every PR

Until now the test suite was authoritative only when run locally;
nothing stopped a commit landing with `yarn test` red. Wire up a
GitHub Actions workflow that gates push + pull_request on both
`yarn typecheck` and `yarn test --run`.

The setup mirrors CLAUDE.md's "Setup walkthrough":
- Check the client into `<workspace>/Nitro-V3`.
- Check `duckietm/Nitro_Render_V3` as a sibling at
  `<workspace>/Nitro_Render_V3`, since the build / typecheck wire
  the renderer in via filesystem aliases that expect that layout.
- Symlink `Nitro-V3/node_modules/@nitrots/nitro-renderer` →
  `../../../Nitro_Render_V3` so `tsconfig.json`'s `include`
  entry pointing at `node_modules/@nitrots/nitro-renderer/src/**/*.ts`
  actually resolves.
- `yarn install --frozen-lockfile` in both repos, then run
  `yarn typecheck` and `yarn test --run` in the client.

Node 22 (matches the local toolchain). Yarn classic, with the
workflow's `setup-node` caching the `yarn.lock` of both repos so
subsequent runs don't reinstall from scratch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -299,7 +299,10 @@ Fix shapes documented; both are reasonable PRs on their own.
 - **Skip-motivated god-hook splits are fine** — when a hook's actions
   mutate internal state, document the reason in the commit message and
   move on rather than forcing a bad split.
-- **`yarn test` must stay green** on every commit. Currently 113/113.
+- **`yarn test` must stay green** on every commit. Currently 163/163.
+  The GitHub Actions workflow at `.github/workflows/ci.yml` runs
+  `yarn typecheck` + `yarn test --run` on every push to `main` /
+  `feat/**` and on every PR — both must pass.
 - **Lint baseline**: don't regress. Some pre-existing errors (`FC<{}>`,
   `IMessageEvent | undefined` redundant union in the local sandbox where
   the renderer SDK isn't installed) are out of scope here.