ci: gate the vitest suite in CI

[myobie]

Gate the vitest suite in CI

The only CI on push/PR was nix build (.github/workflows/nix.yml) — the
vitest suite never ran in CI. That is how two breaks reached main unseen,
both green under nix build but red under npm test:

1. feat: exit-time cleanup of dead non-permanent sessions #111 exit-reap × parity Add Nix flake and fish shell completions #1 — feat: exit-time cleanup of dead non-permanent sessions #111 (exit-time cleanup of dead
   non-permanent sessions) makes default sessions reap themselves at exit, so
   peek-after-exit returns "not found". fix(cli): run --force creates a nested session inside a pty (parity R1 #4) #112's parity Add Nix flake and fish shell completions #1 (asserting the
   pre-feat: exit-time cleanup of dead non-permanent sessions #111 post-exit persistence) merged right after and now fails. Neither
   PR's CI saw the other, and nix build never ran either test.
2. Add pty completions <shell> generator (fish/bash/zsh, SSOT, dynamic sessions) #106 completions help-docs drift — the completions dispatch case was
   never added to the help docs; help.test.ts's "every dispatch case is
   documented" check fails.

What this adds

.github/workflows/test.yml: on pull_request + push-to-main →
checkout → setup-node@v4 (Node 22, matching flake.nix nodejs_22) →
npm ci → npm run build (the suite exercises dist/cli.js + dist/server.js)
→ npm test.

⚠️ This gate is RED on current main — by design

It will fail until the 4 known vitest failures are fixed (3 post-exit parity

• 1 completions). Those fixes are pending product/routing decisions
  (reap-vs-preserve default; completions docs) — tracked separately. A red check
  here is precisely the gate doing its job: catching what nix build couldn't.
  Merge once the underlying fixes make it green.

Scope kept minimal (vitest gate only, as requested). A typecheck job could be
a small follow-up.

[myobie]

Superseded by #114 — the vitest CI gate is folded into that hotfix PR (which also un-breaks the parity contract + adds the shell/claude-stub setup the gate needs to go green). Closing this standalone gate PR.