Changesets: have single git tag on publish

Presently when we publish, we have separate tags for each package like `@vX.Y.Z`. Since we're using `fixed` for all packages, we'd like to just have one git tag on publish of `vX.Y.Z`.

## Options

### Don't use changesets

We could do something like:

```sh
$ pnpm publish -r
# magically infer $VERSION from say the core `package.json:version`
$ git tag -a $VERSION -m $VERSION
$ git push --tags
```

Start by locating the repository's Changesets configuration and publish scripts, then run the existing release flow to observe the package-specific tags. Compare that output with the requested single vX.Y.Z tag; done means a fixed-version publish creates and pushes one tag without separate package tags.

Written by the indexing model from the issue text.