ci: add a vitest gate (full npm test) alongside the nix flake check

[myobie]

What

Adds a vitest CI gate — a Test workflow that runs the full npm test (vitest) on PRs + push-to-main, closing the gap you flagged: convoy CI ran only nix flake check (build + help/completions/typecheck + the completions-parity test), never the full vitest suite. So a red vitest test could ship green. (The gap pty closed in #114.)

Why a separate workflow, not a flake check

Most of convoy's vitest suite shells out to git worktree and spawns the real bin/convoy — awkward in the hermetic nix sandbox, which is why only the pure completions-parity test lives in the flake today. A normal runner handles the shell-outs cleanly. nix.yml stays as-is; this runs alongside it.

The convoy-specific wrinkle (vs pty's #114)

convoy depends on @compoundingtech/pty + @compoundingtech/smalltalk via file:../<sibling> paths (the flake links them from flake inputs). So the workflow:

• checks out pty + smalltalk beside convoy and builds them — convoy imports their compiled dist/;
• installs zsh + fish so the generated-completion syntax checks actually run instead of skipping — this is exactly what would have caught the zsh apostrophe bug from convoy eval: batch-job type + completion event + orchestrator → machine verdict (Phase 1) #96 locally;
• uses npm install (not npm ci) for convoy — the sibling repos float ahead of convoy's pinned lockfile, so npm ci would break on any sibling version bump;
• sets a git identity (the cutWorktree tests git commit in throwaway repos);
• Node 24 (convoy needs ≥23.6 for bin/convoy's .ts type-stripping at load; matches the flake's nodejs_24).

The workflow validates itself on this PR.

🤖 Generated with Claude Code

https://claude.ai/code/session_01MCzqQKSpPiNX2ketyubByS