CI does not run on stacked PRs (pull_request trigger restricted to main)

## Context

Noted while reviewing #56, which targets the `fix/napi-out-parameters-v2` branch (stacked on #57). No CI checks ran on it because `.github/workflows/ci.yml` triggers only on:

```yaml
on:
pull_request:
branches: [main]
```

Any PR whose base is another feature branch gets zero checks (build, zig tests, JS tests, lint), so stacked PRs can only be validated locally until the base PR merges or the PR is retargeted.

## Proposal

Drop the branch filter on the `pull_request` trigger (or widen it) so CI runs for PRs against any base branch:

```yaml
on:
pull_request:
push:
branches: [main]
```

Ref: `.github/workflows/ci.yml`